: 


I 


• 


' 

' 


PRINCIPLES  AND  PRACTICE 


OF 


ELECTRICAL  ENGINEERING 


BY 
ALEXANDER    GRAY 

WHIT.  Sen.,  B.  Sc.  (EoiN.  AND  McGiLL) 

ASSISTANT   PROFESSOR   OF    ELECTRICAL    ENGINEERING, 

MCGILL     UNIVERSITY,     MONTREAL,     CANADA; 

AUTHOR  OF  ELECTRICAL  MACHINE  DESIGN 


FIRST  EDITION 


McGRAW-HILL  BOOK  COMPANY,  INC. 

239  WEST  39TH  STREET,  NEW  YORK 

6  BOUVERIE  STREET,  LONDON,  E.  C. 

1914 


COPYRIGHT,  1914,  BY  THE 
MCGRAW-HILL  BOOK  COMPANY,  INC. 


THE. MAPLE. PRESS. YORK. PA 


PREFACE 

The  following  work  is  based  on  a  lecture  and  laboratory  course 
given  to  the  senior  civil,  mechanical,  and  mining  students  at 
McGill  University.  It  is  therefore  suited  for  men  who  desire 
to  obtain  a  broad  idea  of  the  principles  and  practice  of  electrical 
engineering  and  who  have  only  a  limited  amount  of  time  to  spend 
on  the  subject.  For  such  men  it  is  necessary  to  emphasize  the 
fundamental  principles,  and  to  develop  the  subject  by  elaborating 
on  these  principles  rather  than  by  the  solution  of  mathematical 
equations,  because  only  in  this  way  can  the  student  be  given  such 
a  grip  of  the  subject  in  the  short  time  available,  that  he  is  able 
thereafter  to  make  intelligent  use  of  the  data  contained  in  the 
electrical  handbooks,  or  take  up  with  advantage  a  further  study 
of  the  special  treatises  on  the  subject. 

The  book  gives  a  self-contained  lecture  and  laboratory  course. 
The  chapters  on  the  control  and  applications  of  electrical  ma- 
chinery have  been  so  written  that  large  sections  of  these  chapters 
may  be  set  for  private  reading.  In  the  laboratory  course,  com- 
plete references  are  given  to  the  theory  and  purpose  of  each  ex- 
periment, and  these  references  in  no  case  go  beyond  the  text 
contained  in  the  body  of  the  work. 

The  author  wishes  to  acknowledge  his  indebtedness  to  Mr. 
A.  M.  S.  Boyd  and  to  Mr.  R.  Kraus  for  their  help  and  criticism. 

A.  G. 

McGiLL  UNIVERSITY, 
Sept.  I,  1914. 


CONTENTS 

PREFACE    .....    v 

INTRODUCTION  .  .   xxi 


CHAPTER  I 

MAGNETISM  AND  MAGNETIC  UNITS 
Article  Page 

1.  Magnets 1 

2.  Coulomb's  Law . 1 

3.  The  Magnetic  Field 1 

4.  Lines  of  Force _.  _ 2 

5.  Lines  of  Force  from  a  Unit  Pole  .    .                 3 


CHAPTER  II 
ELECTROMAGNETISM 

6.  Direction  of  an  Electric  Current 4 

7.  Magnetic  Field  Surrounding  a  Conductor  Carrying  Current ...  4 

8.  Force  at  the  Center  of  a  Circular  Loop  Carrying  Current      ...  4 

9.  Electromagnets 5 

10.  Force  on  a  Conductor  Carrying  Current  in  a  Magnetic  Field    .    .  6 

11.  Moving  Coil  Ammeters — _ 8 

CHAPTER  III 
ELECTROMAGNETIC  INDUCTION 

12.  Electromagnetic  Induction 9 

13.  The  Direction  of  the  Induced  Electromotive  Force 9 

14.  Mutual  Induction 11 

15.  Self  Induction 11 

CHAPTER  IV 
WORK  AND  POWER 

16.  Transformation  of  Mechanical  into  Electrical  Energy 13 

17.  Unit  of  Work 14 

18.  Heat  Energy  and  Electrical  Energy 15 

19.  Conversion  Factors 15 

20.  Problems  on  Work  and  Power . 15 

vii 


vni  CONTENTS 

CHAPTER  V 

ELECTRIC  CIRCUITS  AND  RESISTANCE 
Article  Page 

21.  The  Flow  of  Electricity : 18 

22.  Ammeters  and  Voltmeters 19 

23.  Resistance  Circuits 19 

24.  Ohm's  Law 20 

25.  Specific  Resistance 20 

26.  Variation  of  Resistance  with  Temperature 21 

27.  Power  Expended  in  a  Resistance 21 

28.  Insulating  Materials 22 

29.  Dielectric  Strength  of  Insulating  Material 22 

30.  Series  and  Parallel  Circuits 22 

31.  Voltage  Drop  in  a  Transmission  Line 23 

CHAPTER  VI 

RHEOSTATS  AND  RESISTORS 

32.  Rheostats 25 

33.  Resistors 26 

34.  Heater  Units 27 

35.  Cast-iron  Grid  Resistance 27 

36.  Carbon  Pile  Rheostat 29 

37.  Liquid  Rheostat 29 

38.  Size  of  a  Rheostat 31 

CHAPTER  VII 
MAGNETIC  CIRCUITS  AND  MAGNETIC  PROPERTIES  OP  IRON 

39.  Magnetic  Field  due  to  a  Solenoid 32 

40.  Permeability 33 

41.  Reluctance  of  a  Magnetic  Circuit 33 

42.  Magnetization  Curves 34 

43.  Residual  Magnetism      35 

44.  Molecular  Theory  of  Magnetism 36 

45.  Hysteresis 36 

CHAPTER  VIII 

SOLENOIDS  AND  ELECTROMAGNETS 

46.  Pull  of  Solenoids 37 

47.  Electric  Hammer 

48.  Variation  of  the  Pull  of  a  Solenoid 

49.  Circuit  Breaker 39 

50.  Laws  of  Magnetic  Pull 40 

51.  Solenoids  with  Long  and  Short  Plungers 

52.  Iron-clad  Solenoids                                             »....  41 


CONTENTS  ix 

Article  Page 

53.  Lifting  and  Holding  Magnets .    .  43 

54.  Saturation  of  a  Magnetic  Circuit 46 

55.  Electromagnetic  Brakes  and  Clutches 46 

56.  Magnetic  Separator  < 47 

CHAPTER  IX 
ARMATURE  WINDINGS  FOR  DIRECT-CURRENT  MACHINERY 

57.  Principle  of  Operation  of  the  Electric  Generator 48 

58.  Gramme  Ring  Winding 48 

59.  Commutator  and  Brushes 50 

60.  Multipolar  Windings 51 

61.  Drum  Windings      -. 52 

62.  Lamination  of  the  Armature  Core 55 

CHAPTER  X 

CONSTRUCTION  AND  EXCITATION  OP  DIRECT-CURRENT  MACHINES 

63.  Multipolar  Construction 56 

64.  Armature  Construction 58 

65.  Commutator 58 

66.  The  Brushes 58 

67.  Poles  and  Yoke 58 

68.  Large  Generators 58 

69.  Excitation 59 

CHAPTER  XI 

THEORY  OF  COMMUTATION 

70.  Commutation 62 

71.  Theory  of  Commutation , 63 

72.  Shifting  of  the  Brushes 63 

73.  Interpole  Machines 65 

74.  Carbon  Brushes 66 

CHAPTER  XII 
ARMATURE  REACTION 

75.  The  Cross-magnetizing  Effect 67 

76.  The  Demagnetizing  Effect • 68 

77.  Effect  of  Armature  Reaction  on  Commutation 68 

CHAPTER  XIII 

CHARACTERISTICS  OF  DIRECT-CURRENT  GENERATORS 

78.  Magnetization  or  No-load  Saturation  Curve 70 

79.  Self  Excitation  71 


x  CONTENTS 

Article  Page 

80.  Regulation   Curve   of   a    Separately   Excited   or   of   a    Magneto 

Generator " 71 

81.  Regulation  Curve  of  a  Shunt  Generator 72 

82.  To  Maintain  the  Terminal  Voltage  Constant 74 

83.  Compound  Generators 74 

84.  The  Regulation  Curve  of  a  Series  Generator 75 

85.  Problem  on  Generator  Characteristics 76 

CHAPTER  XIV 
THEORY  OF  OPERATION   OF   DIRECT-CURRENT   MOTORS 

86.  Driving  Force  of  a  Motor. .' 78 

87.  Driving  and  Retarding  Forces  in  Generators  and  Motors  ....  78 

88.  The  Back  E.M.F 79 

89.  Theory  of  Motor  Operation 80 

90.  Speed  and  Torque  Formula? 82 

91.  Improvement  of  Commutation  by  Shifting  of  the  Brushes.    ...  83 

92.  Armature  Reaction  in  Generators  and  Motors 83 

CHAPTER  XV 

CHARACTERISTICS  OF  DIRECT-CURRENT  MOTORS 

93.  The  Starting  Torque 85 

94.  The  Starting  Resistance    .    .    : 85 

95.  Motor  Starter 87 

96.  No-voltage  Release 87 

97.  Load  Characteristics 88 

98.  Effect  of  Armature  Reaction  on  the  Speed 89 

§9.  Variable  Speed  Operation 89 

100.  The  Starting  Torque      - 90 

101.  The  Starting  Resistance 91 

102.  Load  Characteristics      92 

103.  Speed  Adjustment      , 92 

104.  The  Compound  Motor 93 

CHAPTER  XVI 

LOSSES,  EFFICIENCY  AND  HEATING 

105.  Mechanical  Losses  in  Electrical  Machinery 95 

106.  Copper  Losses .    . 95 

107.  Hysteresis  Loss 95 

108.  Eddy  Current  Loss - 96 

109.  Stray  Loss 96 

110.  The  Efficiency  of  a  Machine 97 

111.  Heating  of  Electrical  Machinery 99 

112.  Permissible  Temperature  Rise 99 


CONTENTS  xi 

CHAPTER  XVII 

MOTOR  APPLICATIONS 

Article  Page 

113.  Limits  of  Output     .    .    .    . 100 

114.  Open,  Semi-enclosed  and  Totally  Enclosed  Motors 100 

115.  Intermittent  Ratings 101 

116.  Effect  of  Speed  on  the  Cost  of  a  Motor 101 

117.  Choice  of  Type  of  Motor 101 

118.  Line  Shaft  Drive 102 

119.  Wood-working  Machinery 102 

120.  Reciprocating  Pumps 103 

121.  Traction  Motors 103 

122.  Crane  Motors      103 

123.  Express  Passenger  Elevators 103 

124.  Shears  and  Punch  Presses 103 

CHAPTER  XVIII 

ADJUSTABLE  SPEED  OPERATION  OF  DIRECT-CURRENT  MOTORS 

125.  Speed  Variation  of  Shunt  Motors  by  Armature  Control     ....  105 

126.  Speed  Variation  of  Shunt  Motors  by  Field  Control     ......  107 

127.  Speed  Regulation  of  an  Adjustable  Speed  Shunt  Motor 107 

128.  Electric  Drive  for  Lathes  and  Boring  Mills 108 

129.  Multiple  Voltage  Systems 109 

130.  Ward  Leonard  System       110 

131.  Drive  for  Ventilating  Kans Ill 

132.  Armature  Resistance  for  Speed  Reduction 112 

133.  Motors  for  Small  Desk  Fans -s- 112 

134.  Printing  Presses      113 

CHAPTER  XIX 
HAND-OPERATED  FACE  PLATE  STARTERS  AND  CONTROLLERS 

135.  Knife  Switches ' 114 

136.  Auxiliary  Carbon  Contacts       114 

137.  Blow-out  Coils    . 115 

138.  Horn  Gaps       116 

139.  Fuses 117 

140.  Circuit  Breakers 117 

141.  Motor  Starters ......  117 

142.  The  Sliding  Contact  Type  of  Starter ..117 

143.  Starting  Resistance , 118 

144.  Overload  Release 118 

145.  Multiple  Switch  Starters .119 

146.  Compound  Starters 120 

147.  Speed  Regulators 121 

148.  Controllers  for  Series  Motors 122 


xii  ,       CONTENTS 

CHAPTER  XX 

DRUM  TYPE  CONTROLLERS 

Article  Page 

149.  Drum  Type  Controllers ' 124 

150.  No-voltage  and  Overload  Release 125 

151.  Street  Car  Controller  for  Series  Parallel  Control 126 

152.  Reversing  Drum 129 

153.  Mechanical  Features  of  Drum  Controllers 129 

CHAPTER  XXI 
AUTOMATIC  STARTERS  AND  CONTROLLERS 

154.  Automatic  Solenoid  Starter 130 

155.  Float  Switch  Control 130 

156.  Magnetic  Switch  Controller 131 

157.  Multiple  Unit  Control  of  Railway  Motors 133 

158.  Automatic  Magnetic  Switch  Starters 133 

159.  Automatic  Starter  with  Series  Switches 135 

CHAPTER  XXII 
ELECTROLYSIS  AND  BATTERIES 

160.  Electrolysis 139 

161.  Voltameter  ....                                                                            .    .  139 

162.  Electric  Battery 140 

163.  Theory  of  Battery  Operation " 140 

164.  Polarization 141 

165.  The  E.M.F.  and  Resistance  of  Cells 141 

166.  The  Daniell  Cell 141 

167.  Calculation  of  the  E.M.F.  of  a  Daniell  Cell  .                                    .  142 

168.  Local  Action 143 

169.  Leclanche  Cell 143 

170.  Dry  Cells .143 

171.  Edison  Lalande  Cell  .    . 

172.  Power  and  Energy  of  a  Battery 144 

173.  Battery  Connections 144 

CHAPTER  XXIII 

STORAGE  BATTERIES 

174.  Action  of  the  Lead  Cell 146 

175.  Storage  or  Secondary  Battery .  147 

176.  Sulphation 147 

177.  Construction  of  the  Plates 148 

178.  Construction  of  a  Lead  Battery 149 

179.  Voltage  of  a  Lead  Battery .151 

180.  Capacity  of  a  Cell 153 


CONTENTS  xiii 

Article  Page 

181.  Ampere-hour  Efficiency 153 

182.  Watt-hour  Efficiency ' 154 

183.  Effect  of  Temperature  on  the  Capacity 155 

184.  Limit  of  Discharge 155 

185.  Treatment  of  Lead  Cells 156 

186.  Action  of  the  Edison  Battery .    .  157 

187.  Construction  of  the  Plates 158 

188.  Construction  of  an  Edison  Battery 158 

189.  The  Voltage  of  an  Edison  Battery 160 

190.  Characteristics  of  an  Edison  Battery 160 

CHAPTER  XXIV 
OPERATION  OF  GENERATORS 

191.  Operation  of  the  Same  Shunt  Machine  as  a  Generator  or  as  a  Motor  162 

192.  Loading  Back  Tests 163 

193.  Parallel  Operation 164 

194.  Shunt  Generators  in  Parallel 164 

195.  Division  of  Load  among  Shunt  Generators  in  Parallel 165 

196.  Compound  Generators  in  Parallel 166 

197.  'Division  of  Load  among  Compound  Generators 167 

CHAPTER  XXV 
OPERATION  OF  GENERATORS  AND  BATTERIES  IN  PARALLEL 

198.  Isolated  Lighting  Plants 169 

199.  Lighting  Plants  for  Farm  Houses 169 

200.  Lamp  Circuit  Regulator ~7~ 170 

201.  Small  Isolated  Power  Stations 171 

202.  Resistance  Control 171 

203.  End  Cell  Control 172 

204.  Booster  Charge,  End  Cell  Discharge 173 

205.  Capacity  of  Battery 175 

206.  Batteries  for  Rapidly  Fluctuating  Loads 176 

207.  The  Differential  Booster • 176 

208.  Carbon  Pile  Regulator ' ...  176 

209.  Floating  Batteries 179 

CHAPTER  XXVI 

CAR  LIGHTING  AND  VARIABLE  SPEED  GENERATORS 

210.  Systems  of  Vehicle  Lighting 181 

211.  Straight  Storage  for  Trains 181 

212.  Head  and  End  System 181 

213.  Carbon  Pile  Lamp  Regulator 182 

214.  The  Axle  Generator  Systems 182 

215.  Automatic  Switch  .                                                                                .  183 


xiv  CONTENTS 

Article  Page 

216.  Generator  Regulator 183 

217.  Pole  Changer • 184 

218.  The  Stone  Generator 185 

219.  Lighting  Generators  for  Motor  Cars   . 186 

220.  Constant  Speed  Generators 186 

221.  Bucking  Field  Coils 186 

222.  Vibrating  Contact  Regulator 187 

223.  The  Rosenberg  Generator 188 

CHAPTER  XXVII 

ALTERNATING  VOLTAGES  AND  CURRENTS 

224.  The  Simple  Alternator 191 

225.  The  Wave  Form 193 

226.  The  Oscillograph 193 

227.  Frequency 194 

228.  Vibrating  Reed  Type  of  Frequency  Meter    . 195 

229.  Average  Value  of  Current  and  Voltage 197 

230.  The  Heating  Effect  of  an  Alternating  Current 197 

231.  Symbols 198 

232.  Voltmeters  and  Ammeters  for  Alternating-current  Circuits    .    .    .  198 

CHAPTER  XXVIII 
REPRESENTATION  OF  ALTERNATING  CURRENTS  AND  VOLTAGES 

233 200 

234.  Electrical  Degrees 200 

235.  Vector  Representation  of  Alternating  Voltages  and  Currents    .    .  201 

236.  The  Sum  of  Two  Alternating  Voltages  of  the  Same  Frequency .    .  203 

CHAPTER  XXIX 

INDUCTIVE  CIRCUITS 

237.  Inductance  . 205 

238.  Make  and  Break  Spark  Ignition 206 

239.  The  Coefficient  of  Self  Induction 206 

240.  Alternating  Currents  in  Inductive  Circuits 207 

241.  Voltage  and  Current  Relations 208 

242.  Power  in  an  Inductive  Circuit 209 

243.  Examples  of  Inductive  and  Non-inductive  Circuits 209 

244.  Voltage,  Current  and  Power  in  Res :stance  Circuits 211 

245.  Resistance  and  Inductance  in  Series .  212 

246.  The  Power  Factor 213 

247.  The  Wattmeter 214 

248.  Transmission  Line  Regulation  and  Losses 215 

249.  Resistance  and  Inductance  in  Parallel    .                                            .  216 


CONTENTS  xv 

CHAPTER  XXX 

CAPACITY  CIRCUITS 

Article  Page 

250.  Condensers 218 

251.  Capacity  Circuits  with  Direct  and  with  Alternating  Currents  .    .  219 

252.  Phase  Relation  between  Voltage  and  Current  in  Capacity  Circuits  .  220 

253.  Voltage  and  Current  Relations  in  Capacity  Circuits 221 

254.  Parallel  Plate  Condenser 222 

255.  Power  in  Capacity  Circuits 223 

256.  The  Formula?  Used  in  Circuit  Problems 223 

257.  Resistance,  Inductance  and  Capacity  in  Series 224 

258.  Resistance,  Inductance  and  Capacity  in  Parallel 226 

CHAPTER  XXXI 

ALTERNATORS 

259.  Alternator  Construction 229 

260.  Two-phase  Alternator 230 

261.  Three-phase  Alternators 231 

262.  Y-Connection 233 

263.  Delta-connection 233 

264.  Voltages,  Currents  and  Power  in  a  Y-Connected  Machine     .    .    .  235 

265.  Voltages,  Currents  and  Power  in  a  Delta-connected  Machine    .    .  236 

266.  Connection  of  a  Three-phase  Load      237 

267.  Power  Measurement  in  Polyphase  Circuits 238 

268.  Alternator  Construction ' 239 

269.  The  Revolving  Armature  Type  of  Alternator 240 

270.  The  Inductor  Alternator ^ 241 

271.  Magneto  Alternators • 242 

CHAPTER  XXXII 
ALTERNATOR  CHARACTERISTICS 

272.  Armature  Reaction 244 

273.  Vector  Diagram  at  Full-load 245 

274.  Regulation  Curves  of  an  Alternator 245 

275.  Experimental  Determination  of  Alternator  Reactance    ......  246 

276.  Automatic  Regulators 249 

277.  Efficiency 250 

278.  Rating  of  Alternators .' 251 

CHAPTER  XXXIII 
SYNCHRONOUS  MOTORS  AND  PARALLEL  OPERATION 

279.  Principle  of  Operation  of  Synchronous  Motors 252 

280.  The  Back  E.M.F.  of  a  Synchronous  Motor 253 

281.  Mechanical  Analogy 254 


xv  rav'/'A'AY'N 

Article  Page 

282.  Vector  Diagram  for  a  Synchronous  Motor 254 

283.  Maximum  Output 255 

284.  Operation  of  a  Synchronous  Motor  when  Under-  and  Over-excited  256 

285.  Use  of  the  Synchronous  Motor  for  Power  Factor  Correction    .    .    256 

286.  Synchronizing 258 

287.  Hunting 258 

288.  Parallel  Operation  of  Alternators 259 

CHAPTER  XXXIV 
TRANSFORMER  CHARACTERISTICS 

289.  The  Transformer 261 

290.  Constant  Potential  Transformer  . 261 

291.  Vector  Diagram  for  a  Transformer 263 

292.  Induction  Furnace 263 

293.  Leakage  Reactance 264 

294.  Leakage  Reactance  in  Standard  Transformers  and  in  Induction 

Furnaces 266 

295.  The  Constant-current  Transformer     .    .    . 267 

296.  The  Efficiency  of  a  Transformer 268 

297.  Hysteresis  Loss 269 

298.  Eddy  Current  Loss 269 

299.  Iron  Losses 269 

300.  The  All-day  Efficiency 269 

301.  Cooling  of  Transformers 270 

CHAPTER  XXXV 

TRANSFORMER  CONNECTIONS 

302.  Lighting  Transformers 273 

303.  Connections  to  a  Two-phase  Line 273 

304.  Connections  to  a  Three-phase  Line .    .    . 276 

305.  Advantages  and  Disadvantages  of  the  Y-  and  Delta-connection .  278 

306.  Types  of  Transformer 279 

307.  The  Autotransf ormer 279 

308.  Boosting  Transformers  and  Feeder  Regulators 281 

CHAPTER  XXXVI 
POLYPHASE  INDUCTION  MOTORS 

309.  The  Induction  Motor 283 

310.  The  Revolving  Field , .284 

311.  The  Revolving  Field  of  a  Three-phase  Motor  . 

312.  Multipolar  Machines 287 

313.  The  Starting  Torque .287 

314.  The  Wound  Rotor  Motor .  - 288 


CONTENTS  xvii 

Article  Page 

315.  Running  Conditions 290 

316.  Vectar  Diagrams  for  the  Induction  Motor 291 

317.  Adjustable  Speed  Operation .    .  293 

318.  Induction  Generator 294 

319.  Self-starting  Synchronous  Motors 294 

320.  Dampers  for  Synchronous  Machines 295 

CHAPTER  XXXVII 
,     INDUCTION  MOTOR  APPLICATIONS  AND  CONTROL 

321.  Choice  of  Type  of  Motor 296 

322.  Line  Shaft  Drfve 297 

323.  Wood-working  Machinery 297 

324.  Cement  Mills ....  297 

325.  Motors  for  Textile  Machinery 298 

326.  Adjustable  Speed  Motors 298 

327.  Crane  Motors 298 

328.  Shears  and  Punch  Presses 299 

329.  Adjustable  Speed  Service 299 

330.  Resistance  for  Adjustable  Speed  Motors 300 

331.  Switches  for  Alternating-current  Circuits 300 

332.  Starting  of  Squirrel-cage  Induction  Motors  . 301 

333.  Starting  Compensator '. 302 

334.  The  Star-delta  Method  of  Starting 304 

335.  Starter  for  a  Wound  Rotor  Motor 305 

336.  Automatic  Starters .......  306 

-    CHAPTER  XXXVIII 
SINGLE-PHASE  MOTORS 

337.  Single-phase  Induction  Motors    .    . 308 

338.  Split-phase  Method  of  Starting 308 

339.  Running  Torque  of  a  Single-phase  Motor 308 

340.  Single-phase  Series  Motor , 310 

341.  Armature  Reaction 312 

342.  The  Repulsion  Motor 313 

343.  Commutation  of  Series  and  Repulsion  Motors 314 

344.  Wagner  Single-phase  Motor 314 

CHAPTER  XXXIX 

MOTOR-GENERATOR  SETS  AND  ROTARY  CONVERTERS 

345.  Motor-generator  Set 315 

346.  The  Booster  Set 315 

347.  The  Balancer  Set 316 

348.  Three-wire  Generator 317 

349.  To  Transform  from  Alternating  to  Direct  Current 318 


xviii  CONTENTS 

Article  Page 

350.  Rotary  Converter 318 

351.  Motor-generator  Sets  and  Rotary  Converters 319 

352.  Polyphase  Rotary  Converter 320 

353.  Split-pole  Rotary  Converter -.    . 320 

354.  Frequency  Changers 321 

CHAPTER  XL 

ELECTRIC  TRACTION 

355.  Tractive  Effort 322 

356.  Speed  Time  Curve 323 

357.  Energy  Required  by  a  Car 326 

358.  Characteristics  Desired  in  Railway  Motors 327 

359.  Motor  Construction 329 

360.  Distribution  to  the  Cars 329 

361.  Alternating-  and  Direct-current  Traction 329 

362.  Motor  Car  Trains 332 

363.  Electric  Locomotives 332 

364.  Crane  and  Hoist  Motors 332 

365.  Braking 333 

366.  Flywheel  Motor-generator  Sets  for  Mine  Hoisting 335 

367.  Safety  Devices 337 

CHAPTER  XLI 
TRANSMISSION   AND  DISTRIBUTION 

368.  Direct-current  Stations *.....   338 

369.  Alternating-current  Stations 339 

370.  The  Voltages  Used  in  Practice 341 

371.  Comparison  between  Single-phase  and  Three-phase  Transmission.  341 

372.  Lightning  Arresters 343 

373.  Switches 345 

374.  Overhead  Line  Construction 346 

375.  Underground  Construction 347 

376.  Switchboards 348 

377.  Instrument  Transformers 351 

CHAPTER  XLII 
ELECTRIC  LIGHTING 

378.  The  Carbon  Incandescent  Lamp 352 

379.  The  Tungsten  Lamp 352 

380.  Gas-filled  Tungsten  Lamp 353 

381.  The  Unit  of  Light 354 

382.  Arc  Lamps 354 

383.  The  Direct-current  Open  Arc 355 

384.  Direct-current  Enclosed  Arc.  .  355 


CONTENTS  xix 

Article  Page 

385.  Alternating-current  Enclosed  Arc 356 

386.  Flame  Arc  Lamps 356 

387.  Luminous  Arc  Lamp 357 

388.  Mercury  Vapor  Converter    .    .    .    , 357 

389.  Mercury  Vapor  Lamp . 359 

390.  Shades  and  Reflectors 359 

391.  Efficiency  of  Illuminants 360 

392.  Light  and  Sensation 361 

393.  Reflection  and  Color v    .    .  362 

394.  Principles  of  Illumination 362 

395.  Quality  of  the  Light 363 

396.  Glare 363 

397.  Shadows 363 

398.  Intensity  of  Illumination 364 

399.  Lines  of  Illumination 364 

400.  Power  Distribution  for  Lighting .    .  365 

CHAPTER  XLIIT 

LABORATORY  COURSE 

401.  Protection  of  Circuits.   . 368 

402.  Ammeter  Shunts .  368 

403.  Safe  Carrying  Capacity  of  Copper  Wires 368 

404.  Control  of  the  Current  in  a  Circuit 369 

Exp.  1.  Measurement  of  the  Resistance  of  the  Field  Coil  Circuit  .  .  .  370 
Exp.  2.  Measurement  of  the  Resistance  of  the  Armature  Circuit  .  .  370 
Exp.  3.  Speed  Adjustment  of  a  Direct-current  Shunt  Motor  ....  371 

Exp.    4.  Voltage  of  a  Direct-current  Generator 371 

Exp.    5.  Regulation  of  Direct-current  Generators 372 

Exp.    6.  Brake  Tests  on  Direct-current  Motors 373 

Exp.    7.  Starting  Torque  Tests  on  Direct-current  Motors      373 

Exp.    8.  Stray  Loss  and  Efficiency  of  a  Direct-current  Motor   ....   374 

Exp.    9.  Heat  Run  on  a  Direct-current  Generator  .    .    . 374 

Exp.  10.  Voltage  Regulation  of  a  Three-wire  System       374 

Exp.  11.  Fuse  Testing 375 

Exp.  12.  Calibration  of  a  Circuit  Breaker 375 

Exp.  13.  Alternating-current  Series  Circuit 376 

Exp.  14.  Predetermination   of  the   Characteristics   of   an   Alternating 

Current  Circuit 376 

Exp.  15.  Characteristics  of  a  Constant  Potential  Transformer  ....   377 

Exp.  16.  Regulation  of  an  Alternator 377 

Exp.  17.  Starting    and    Running    Characteristics    of    a    Synchronous 

Motor 378 

Exp.  18.  Characteristics  of  a  Rotary  Converter 379 

Exp.  19.  Starting  and  Running  Characteristics  of  a  Polyphase  Induc- 
tion Motor 380 

Exp.  20.  Transformer  Connections 381 

INDEX  ,   383 


INTRODUCTORY 

Before  a  study  of  electric  circuits  and  machinery  can  be 
made,  it  is  necessary  to  define  the  electric  and  the  magnetic 
units  arid  express  them  in  terms  of  the  fundamental  units  and 
derived  mechanical  units  which  are  given  below. 

Quantity  Practical  units  Practical  units 

c.g.s.  system  ft.  Ib.  sec.  system 

Length          1  cm.  1  ft  =  30.48  cm. 

Mass  1  gm.  1  Ib  =  453.6  gm. 

Time  1  sec.  1  sec. 

Force  1  dyne  1  poundal  =  1/32.2  Ib. 

1  gm.   =  981  dynes       1  Ib.  =  4.448  X  105  dynes 
Work  or 

energy      1  erg  =  1  dyne  cm.  1  ft.  Ib.  =  1.356    X  107  ergs 
Power  1  erg  per  sec.  1  h.p.  =  550  ft.  Ib.  per  sec. 

=  746  X  107  ergs  per  sec. 

In  the  first  few  chapters  of  this  work  some  of  the  fundamental 
principles  of  electricity  and  magnetism  are  briefly  discussed. 
Parts  of  these  chapters  are  difficult  and  are  of  theoretical  im- 
portance only.  These  are  printed  in  small  type  and  may  be 
omitted  if  the  student  is  willing  to  consider  as  experimental 
laws  what  are  really  laws  depending  on  the  definitions  of  the 
electric  and  the  magnetic  units  and  on  their  interrelations. 


XXI 


PRINCIPLES  AND  PRACTICE 

OF 

ELECTRICAL  ENGINEERING 

CHAPTER  I 
MAGNETISM  AND  MAGNETIC  UNITS 

1.  Magnets. — The  power  of  a  magnet  to  attract  or  repel  is 
concentrated  at  certain  points  called  poles.     A  simple  magnet 
has  two  poles  which  are  equal  and  opposite  and  the  line  joining 
them  points  north  and  south  when  the  magnet  is  allowed  to 
swing  freely  in  a  horizontal  plane.     The  pole  pointing  toward 
the  north  is  called  the  north  (N)  pole,  that  pointing  toward 
the  south  is  called  the  south  ($)  pole. 

Like  poles  repel  one  another,  unlike  poles  attract  one  another. 

2.  Coulomb's  Law  states  that  the  force  between  two  magnetic  poles  is 
directly  proportional  to  the  strengths  of  the  poles  and  inversely  proportional 
to  the  square  of  the  distance  between  the  poles,  thus,  in  Fig.  1, 


FIG.  1. 


where  /  is  the  force  between  the  poles, 

r  is  the  distance  between  the  poles, 

m  and  mi  are  the  strengths  of  the  poles, 

k  is  a  constant  which  depends  on  the  surrounding  medium  and  on  the 

units  chosen. 
The  c.g.s.  unit  of  pole  strength  is  chosen  so  as  to  make  k  =  1  when/  is  in 

dynes,  r  in  cm.  and  the  medium  is  air,  then/  =  —  —  . 

A  unit  pole  therefore  acts  on  an  equal  pole  in  air,  at  a  distance 
of  1  cm.  from  it,  with  a  force  of  1  dyne. 

3.  The  magnetic  field  is  the  name  given  to  the  space  surround- 
ing a  magnet,  but  is  limited  in  practice  to  the  space  within  which 
the  force  of  the  magnet  is  perceptible.  A  magnetic  pole  placed 
in  a  magnetic  field  is  acted  on  by  a  force  which  is  proportional 

1 


2  PRINCIPLES  OF  ELECTRICAL  ENGINEERING     [CHAP,  i 

to  the  strength  of  the  magnetic  pole  and  to  the  strength  or  in- 
tensity of  the  magnetic  field. 

The  intensity  of  a  magnetic  field  at  any  point  is  taken  as  the 
force  in  dynes  on  a  unit  pole  at  that  point ;  therefore,  a  unit  field 
will  act  on  a  unit  pole  in  air  with  a  force  of  1  dyne. 

The  direction  of  a  magnetic  field  at  any  point  is  taken  as  the 
direction  of  the  force  on  a  north  pole  at  that  point. 


FIG.  2. — Direction  of  the  field  of  a  magnet. 

Let  NS,  Fig.  2,  be  a  magnet  of  pole  strength  m,  and  n  a  unit  north  pole. 
The  pole  N  of  the  magnet  repels  the  unit  pole  with  a  force  =  ra/Yi2  dynes, 
represented  in  magnitude  and  direction  by  the  line  na]  the  pole  S  of  the  mag- 
net attracts  the  unit  pole  with  a  force  =  m/r22  dynes,  represented  in  magni- 
tude and  direction  by  the  line  rib]  the  resultant  force  on  the  unit  pole,  which 
is  a  measure  of  the  field  intensity,  is  represented  in  magnitude  and  direction 
by  the  line  nc. 


FIG.  .  3. — Lines  of  force  surrounding  a  bar  magnet. 

4.  Lines  of  Force. — In  dealing  with  magnetic  problems  it 
is  found  convenient  to  represent  the  magnetic  field  diagram- 
matically  by  what  are  called  lines  of  force.  These  are  con- 
tinuous lines  whose  direction  at  any  point  in  the  field  is  that  of 
the  force  on  a  north  pole  placed  at  the  given  point.  The  number 
of  lines  crossing  1  sq.  cm.  placed  perpendicularly  to  this  direction 
is  made  proportional  to  the  field  intensity  at  the  point  and  unit 
magnetic  field  is  represented  by  one  line  per  sq.  cm. 


ART.  5] 


MAGNETISM  AND  MAGNETIC  UNITS 


In  Fig.  3  the  intensity  of  the  magnetic  field  is  greatest  at  the 
poles  and  decreases  as  the  distance  from  the  poles  increases,  so 
that  the  lines  of  force  which  represent  this  field  spread  out  from 
the  poles  as  shown.  Since  a  north  pole  n  placed  in  this  field  is 
repelled  by  the  pole  N  and  attracted  by  the  pole  S,  the  lines  of 
force,  being  drawn  in  the  direction  of 
the  force  on  a  north  pole  placed  in  the 
field,  must  leave  the  N  pole  and  enter 
the  S  pole. 

The  total  number  of  lines  of  force 
leaving  or  entering  a  magnetic  pole  is 
called  its  magnetic  flux  <£. 

The  flux  density  (B  at  any  point  in 
a  magnetic  field  is  the  number  of  lines 
of  force  crossing  unit  area  placed  per- 
pendicular to  the  direction  of  the  lines 
of  force  at  that  point. 


FIG.  4. 


5.  Lines  of  Force  from  a  Unit  Pole. — If  a  unit  pole  were  surrounded  by  a 
sphere  of  1  cm.  radius,  as  in  Fig.  4,  another  unit  pole  placed  on  the  surface  of 
this  sphere  would  be  acted  on  with  unit  force  and  so  the  field  intensity  at 
this  surface  must  be  unity;  there  must  therefore  be  one  line  of  force  per 
sq.  cm.  of  sphere  surface  or  a  total  of  4?r  lines,  as  the  surface  area  of  a  sphere 
of  1  cm.  radius  is  4ir  sq.  cm. 

Since  the  number  of  lines  from  a  unit  pole  is  4n-,  therefore  the  number  from 
a  pole  of  strength  ra  is  4wm. 


CHAPTER  II 
ELECTROMAGNETISM 

6.  Direction  of  an  Electric  Current. — P  and  Q,  Fig.  5,  are 
conductors  carrying  current;  the  current  is  going  down  in  con- 
ductor P  and  coming  up  in  conductor  Q.  Let  the  direction 
of  the  current  be  represented  by  an  arrow;  at  the  end  of  con- 
ductor P  one  would  see  the  tail  of  the  arrow,  represented  by  a 
cross,  while  at  the  end  of  conductor  Q  the  point  of  the  arrow 
would  be  seen,  this  is  represented  by  a  dot. 


Out  In 

FIG.  5. — Direction  of  an  electric  current. 

7.  Magnetic  Field  Surrounding  a  Conductor  Carrying  Current. 
— A  conductor  carrying  current  is  surrounded  by  a  magnetic 
field  represented  by  lines  of  force  as  shown  in  Fig.  6.  To  deter- 


Current  Down  Current  Up 

FIG.  6. — Field  surrounding  a  conductor  carrying  current. 

mine  the  direction  of  these  lines  the  following  rule  is  used:  If 
a  corkscrew  is  screwed  into  the  conductor  in  the  direction  of 
the  current  then  the  head  of  the  corkscrew  has  to  be  turned  in 
the  direction  of  the  lines  of  force. 

8.  Force  at  the  Centre  of  a  Circular  Loop  Carrying  Current. — Fig.  7  shows 
a  wire,  carrying  a  current  i,  and  bent  to  form  a  circular  loop  of  radius  r. 
The  direction  of  the  magnetic  field  produced  is  found  by  the  rule  in  the  last 
paragraph. 

4 


ART.  9] 


ELECTROMAGNETISM 


An  element  ab  acts  on  a  unit  pole  at  the  centre  of  the  loop  with  a  force  / 
abXi 


which  is  found  to  be  =  k 
complete  loop 


and  the  total  force  F  on  this  pole  due  to  the 


where  k  is  a  constant  which  depends  on  the  medium  and  on  the  units  chosen. 
The  unit  of  current  is  chosen  so  as  to  make  k  =  1  when  F  is  in  dynes,  r  is 


FIG.  7. — Magnetic  field  produced  by  a  loop  carrying  current. 

in  cm.  and  the  medium  is  air,  then  F  =  —  dynes;  a  unit  current  is  therefore 

of  such  value  that,  when  flowing  in  a  loop  of  1  cm.  radius,  it  acts  on  a  unit 
pole  at  the  centre  of  the  loop  with  a  force  of  2ir  dynes.  This  is  called  the 
c.g.s.  unit  of  current;  the  practical  unit,  called  the  ampere,  is  equal  to  one- 
tenth  of  a  c.g.s.  unit. 

9.  Electromagnets. — The   loop    carrying    current,    shown   in 
Fig.  7,  acts  like  a  magnet  and  is  called  an  electromagnet.     The 


N- 


FIG.  8.— The  polarity  of  an  electromagnet. 

strength  of  an  electromagnet  may  be  increased  by  increasing 
the  current  or,  as  in  Fig.  8,  by  increasing  the  number  of  turns. 
The  direction  of  the  magnetic  field  may  be  conveniently  found  by 
another  corkscrew  law  which  states  that  if  the  head  of  the  work 
screw  is  turned  in  the  direction  of  the  current  then  the  screw- 


6  PRINCIPLES  OF  ELECTRICAL  ENGINEERING      [CHAP.II 

itself  will  move  into  the  magnetic  field  in  the  direction  of  the  lines 
of  force;  the  direction  of  the  field  produced  by  the  right-hand 
spiral  in  diagram  A  is  the  same  as  that  produced  by  the  left- 
hand  spiral  in  diagram  B;  this  direction  may  be  reversed  by 
reversing  the  current. 

10.  Force  on  a  Conductor  Carrying  Current  in  a  Magnetic  Field. — In 
Fig.  9,  the  unit  po>le  n  is  acted  on  by  the  current  in  the  loop  with  a  force  of 


FIG.  9.  —  Force  on  a  conductor  carrying  current  in  a  magnetic  field. 

2iri/r  dynes  (page  5)  at  right  angles  to  the  plane  of  the  paper,  where  i 
is  the  current  in  c.g.s.  units.  The  loop  itself  must  be  reacted  on  by  the  unit 
pole  with  an  equal  force  in  the  opposite  direction. 

The  flux  density  (B  at  the  wire,  in  lines  per  sq.  cm.,  due  to  the  unit  pole 
_  flux  from  the  unit  pole 

surface  of  a  sphere  of  r  cm.  radius 

1 


As  shown  above,  the  force  acting  on  the  wire  in  dynes 


=  -2  X  27rr  X  i 


Since  (B  =  - 


ART.  10] 


ELECTROMAGNETISM 


where  (B  is  the  flux  density  at  the  wire  in  lines  per  sq.  cm.,  L  is  the  length  of 
wire  that  is  in  the  magnetic  field  in  cm.  =  2irr  in  the  case  of  a  circular  loop 
i  is  the  current  in  the  wire  in  c.g.s.  units. 

When  a  conductor  is  carrying  current  and  is  in  a  magnetic 
field,  as  in  Fig.  10,  it  is  acted  on  by  a  force  which  is  proportional 


FIG.  10. — Force  on  a  conductor  carrying  current  in  a  magnetic  field. 
Left  Hand  Ruel:  thumb — force;  forefinger — lines  of  force;  middle  finger — 
current. 

to  the  current  and  to  the  strength  of  the  field.  The  direction 
of  this  force  may  be  determined  by  what  is  called  the  left-hand 
rule  which  states  that  if  the  thumb,  the  forefinger  and  the 


FIG.  11. — Moving  coil  ammeter. 


middle  finger  of  the  left  hand  are  placed  at  right  angles  to  one 
another  as  in  Fig.  9  so  as  to  represent  three  coordinates  in  space, 
with  the  thumb  pointed  in  the  direction  of  the  mechanical  force 
and  the  forefinger  in  the  direction  of  the  lines  of  force,  then  the 
middle  finger  will  point  in  the  direction  of  the  current. 


8  PRINCIPLES  OF  ELECTRICAL  ENGINEERING      [CHAP,  n 

11.  Moving  Coil  Ammeters. — The  above  principle  is  applied 
in  one  of  the  most'  satisfactory  types  of  instrument  for  the 
measurement  of  direct  current. 

Such  an  instrument  is  shown  in  Fig.  11.  NS'is  a  permanent 
horseshoe  magnet  with  pole  shoes  bored  out  cylindrically  and 
E  is  a  cylindrical  soft  iron  core  concentric  with  the  pole  faces, 
lines  of  force  therefore  pass  as  shown  in  diagram  A  and  the  flux 
density  in  the  air  gaps  is  uniform.  In  this  magnetic  field  a  coil 
C  is  placed  and  is  supported  on  jewelled  bearings.  The  coil 
consists  of  a  number  of  turns  of  fine  insulated  wire  wound  on  a 
light  aluminium  frame  and  the  current  to  be  measured  is  intro- 
duced to  the  coil  through  the  spiral  springs  D,  diagram  B.  Since 
the  sides  of  the  coil  are  carrying  current  and  are  in  a  magnetic 
field  they  are  acted  on  by  forces  which  turn  the  coil  through  an 
angle  against  the  torsion  of  the  springs  D  and  this  angle  may  be 
read  on  a  scale  over  which  plays  a  pointer  B  attached  to  the 
coil. 


CHAPTER  III 


ELECTROMAGNETIC  INDUCTION 

12.  Electromagnetic  Induction.  —  Faraday's  experiments 
showed  that  when  the  magnetic  flux  threading  a  coil  undergoes  a 
change,  an  electromotive  force  (e.m.f.)  is  generated  or  induced  in 
the  coil  and  that  this  e.m.f.  is  pro- 
portional to  the  time  rate  of  change 
of  the  flux.  If  the  coil  A,  Fig.  12] 
be  moved  from  position  1  where  the 
flux  threading  the  coil  is  0  lines,  to 
position  2  where  the  flu,x  threading 
the  coil  is  zero,  in  a  time  of  t  seconds, 
then  the  average  rate  of  change  of 
flux  is  <j>/t  lines  per  sec. 

The  c.g.s.  unit  of  e.m.f.  is  that 
generated  in  a  coil  of  one  turn  when 
the  flux  threading  the  coil  is  chang- 
ing at  the  rate  of  one  line  per  sec. 
The  practical  unit,  called  the  volt, 
is  equal  to  108  c.g.s.  units  so  that 
when  the  flux  threading  a  coil  of  one 
turn  changes  at  the  rate  of  <f>  lines 
in  t  seconds  the  average  e.m.f.  in- 

duced in  the  coil  =  -  10  ~8  volts  and 
t 

the  e.m.f.   at  any  instant  =  -T.-  10~8     FIG.  12.—  Generation  of  elec- 

tromotive force.     Right  Hand 
volts.  Rule:     thumb  —  motion;  fore- 


That  portion  of  the  coil  wherein  the 
e.m.f.  is  actually  induced  is  the  con- 
ductor xy  which  cuts  the  lines  of  force,  and  the  quantity  dfyjdi  is 
the  rate  at  which  the  lines  are  cut. 

13.  The  direction  of  the  induced  electromotive  force  may 
be  determined  by  Fleming's  three-finger  right-hand  rule  which 
states  that  if  the  thumb,  the  forefinger  and  the  middle  finger  of 
the  right  hand  be  placed  at  right  angles  to  one  another  so  as  to 

9 


10 


PRINCIPLES  OF  ELECTRICAL  ENGINEERING     [CHAP,  in 


represent  three  coordinates  in  space,  with  the  thumb  pointed 
in  the  direction  of  motion  of  the  conductor  relative  to  the 
magnetic  field  and  the  forefinger  in  the  direction  of  the  lines  of 
force,  then  the  middle  finger  will  point  in  the  direction  of  the 
induced  e.m.f. 

The  direction  of  the  current  in  xy,  as  determined  by  the  right-hand  rule,  is 
shown  in  Fig.  13  for  the  case  where  the  coil  is  moving  downward  and  the 
number  of  lines  of  force  threading  the  coil  is  decreasing.  This  current  sets 
up  a  magnetic  flux  <j>c,  the  direction  of  which,  found  by  the  corkscrew  law 
(page  5)  is  the  same  as  that  of  the  main  flux  <£  and  tends  to  prevent  the  flux 
threading  the  coil  from  decreasing. 


FIG.  13.  FIG.  14. 

Direction  of  the  generated  electromotive  force. 

If  now  the  direction  of  motion  of  the  coil  be  reversed  so  that  the  number  of 
lines  of  force  threading  the  coil  is  increasing,  the  current  will  be  reversed,  as 
shown  in  Fig.  14,  and  the  magnetic  flux  <f>e  will  oppose  the  main  flux  <f>  and 
tend  to  prevent  the  flux  threading  the  coil  from  increasing. 

The  general  law  for  the  direction  of  the  induced  e.m.f.  in  a  coil, 
known  as  Lenzs  Law,  states  that  the  induced  e.m.f.  tends  to 
send  an  electric  current  in  such  a  direction  as  to  oppose  the  change 
of  flux  which  produces  it. 

If  the  coil  abed,  Fig.  15,  is  moved  from  ra  to  n,  the  flux  threading 
the  coil  does  not  change  and  the  resultant  e.m.f.  generated  in  the 
coil  is  zero;  the  portions  ab  and  cd  of  the  coil  are  cutting  lines 
of  force  but  the  e.m.fs.  generated  in  these  portions  are  equal 
and  opposite. 


ART.  15] 


ELECTROMAGNETIC  INDUCTION 


11 


14.  Mutual  Induction. — The  flux  threading  a  coil  may  be 
changed  without  moving  the  coil.  Suppose  a  constant  current  is 
flowing  in  the  coil  A,  Fig.  16,  this  produces  a  constant  flux  0  which 
threads  coils  A  and  B  but  noe.m.f.  is  generated  in  coil  B  since  there 


FIG.  15. 

is  no  change  in  the  flux.  If  the  current  in  coil  A  is  increased,  the 
flux  threading  coil  B  will  increase  and  this  change  of  flux  will  in- 
duce an  e.m.f.  in  coil  B  which  will  cause  a  current  72  to  flow  in  such 
a  direction  as  to  oppose  the  increase  in  flux.  If  the  current  in  coil 
A  is  decreased,  the  flux  threading  coil  B  will  decrease  and  this 
change  of  flux  will  induce  in  coil  B  an  e.m.f.  which  will  send  a  cur- 
rent 72  in  such  a  direction  as  to  oppose  the  decrease  in  flux. 


Flux  is 

PL. 

y 
\_ 

L 
"i_ 

i 

PH 

B 

\, 

^ 

Flux  is 

^4 

V 

V 

i_ 

PH 

.c 

i_ 

>-, 

0; 

/2> 

I* 

Increasing 

' 

? 
I 

-\ 

T 

~t 

T 

-\ 

r 

Decreasing 

' 

1 

c 

j 

•* 

-v 

-x 

1 

r 

W 

•w 

FIG.  16. — Direction  of  electromotive  force  of  mutual  induction. 

15.  Self  Induction. — When  the  current  in  a  coil  is  changed,  an 
e.m.f.  is  generated  in  the  coil  itself  in  such  a  direction  as  to  oppose 
the  change  in  the  current.  In  Fig.  17,  for  example,  when  the 
switch  k  is  closed,  the  current  flowing  in  the  coil  does  not  reach  its 
final  value  instantaneously  because,  as  the  current  increases  in 
value,  the  flux  <f>  threading  the  coil  increases  and  causes  an  e.m.f. 
to  be  induced  in  the  coil  in  such  a  direction  as  to  oppose  the  in- 


12          PRINCIPLES  OF  ELECTRICAL  ENGINEERING     [CHAP.III 


crease  of  the  current.     This  opposing  e.m.f.,  called  the  e.m.f.  of 
self  induction,  exists  only  while  the  current  is  changing. 

If,  after  the  current  has  reached  its  final  value,  the  switch  k  is 
suddenly  opened,  the  current  in  the  coil  tries  to  decrease  suddenly 
to  zero  but,  as  it  decreases,  the  flux  threading  the  coil  decreases 
and  causes  an  e.m.f.  to  be  induced  in  the  coil  in  such  a  direction 


Applied  Voltage 


0 

-\ 

•\ 

_ 

7~ 

T 

I 

> 

> 

ft 

FIG.  17. 


foltage  of  Self  Induction 

dtp 
=  a  const,  x  — J-T^ — 


FIG.  18. — Growth  of  current  in  a  coil. 


as  to  oppose  the  decrease  of  the  current;  this  e.m.f.  is  generally 
large  enough  to  maintain  the  current  between  the  switch  contacts 
as  they  are  being  separated  and  accounts  for  much  of  the  flashing 
that  is  seen  when  a  switch  is  opened  in  a  circuit  carrying  current. 
When  the  switch  is  closed,  the  current  increases  to  its  final  value 
as  shown  in  Fig.  18.  As  the  current  i  increases,  the  correspond- 
ing increase  of  the  flux  <£  threading  the  coil  induces  an  e.m.f.  of 
self  induction  which  is  proportional  to  d<j>/dt,  the  rate  of  change 
of  the  flux.  When  the  current  has  ceased  to  change,  the  e.m.f. 
of  self  induction  becomes  zero. 


CHAPTER  IV 
WORK  AND  POWER 

16.  Transformation  of  Mechanical  into  Electrical  Energy.— 

If  the  conductor  xy,  Fig.  19,  be  moved  downward  so  as  to  cut  at 
a  constant  rate  the  lines  of  force  passing  from  N  to  S,  a  constant 
e.m.f.  is  induced  in  the  conductor  ar*d,  by  ad  justing  the  resistance 
R,  the  current  in  the  circuit  may  be  maintained  at  the  value  i 


FIG.  19. 

Right  Hand  Rule  for  generation  of  e.m.f.:  thumb — motion;  forefinger — 
lines  of  force;  middle  finger — e.m.f. 

Left  Hand  Rule  for  direction  of  force:  thumb — force  on  conductor;  fore- 
finger— lines  of  force;  middle  finger — current. 

in  the  direction  shown ;  the  direction  of  the  current  may  be  de- 
termined by  the  right-hand  rule  (page  9).   , 

As  this  conductor  is  carrying  current  in  a  magnetic  field,  it  is 
acted  on  by  a  force  F  the  direction  of  which  may  be  determined 
by  the  left-hand  rule  (page  7).  This  force,  as  shown  in  Fig.  19, 
opposes  the  motion  of  the  conductor  and  hence  mechanical 
energy  must  be  expended  in  moving  the  conductor. 

13 


14          PRINCIPLES  OF  ELECTRICAL  ENGINEERING      [CHAP,  iv 

If  (B  is  the  density  of  the-  magnetic  field  in  lines  per  sq.  cm. 

L  is  the  length  in  cm.  of  that  part  of  the  conductor  which  is  cutting  lines  of 

force 

V  is  the  velocity  of  the  conductor  in  cm.  per  sec. 
i  is  the  current  in  the  conductor  in  c.g.s.  units,  then 
e,  the  e.m.f  .  generated  in  the  conductor  in  c.g.s.  units, 
=  the  lines  of  force  cut  per  sec. 
=  (BL7 

Now  F,  the  force  acting  on  the  conductor   =  (BLi  dynes  (page  6)  and 
the    mechanical    power    in    dyne    cm.    per    sec.    required    to    keep    the 
conductor  moving 
=  FV 


=  ((BLF)t 
=  ei 


=  e 

=  (volts  X  108)  (amperes/  10);  pages  9  and  5 

=  volts  X  amperes  X  107 


The  mechanical  power  required  to  obtain  I  amperes  at  a 
difference  of  potential  of  E  volts  from  an  electrical  machine  which 
has  an  efficiency  of  100  per  cent. 

=  EI  107  ergs  per  sec. 
=  EI  watts 

where  the  watt,  the  practical  unit  of  power,  is  equal  to  107  ergs 
per  second. 

The  power  developed  by  large  electrical  machines  is  expressed 
in  kilowatts,  where  1  kw.  is  equal  to  1000  watts. 

The  horsepower  =  550  ft.  Ib.  per  sec.  . 

=  746  X  107  ergs  per  sec. 
=  746  watts. 

this  result  gives  a  connecting  link  between  the  electrical  and  the 
mechanical  units. 

17.  Unit  of  Work.  —  Work  is  done  when  a  force  is  moved 
through  a  distance.  The  c.g.s.  unit  of  work  is  the  erg,  which 
is  the  work  done  in  moving  a  force  of  1  dyne  through  a  distance 
of  1  cm. 

Power  is  the  rate  at  which  work  is  done  and  is  expressed 
either  in  ergs  per  sec.,  in  watts  (107  ergs  per  sec.),  or  in  horse- 
power (746  X  107  ergs  per  sec.). 

When  the  power,  or  rate  at  which  work  is  being  done,  is  1 
watt,  or  10  7  ergs  per  sec.,  then  the  work  done  in  1  sec. 
is  1  watt-second  or  107  ergs  and  is  jcalled  1  joule.  A  more 


ART.  20]  WORK  AND  POWER  15 

convenient  unit  for  practical  work  is  the  kilowatt-hour  (3600  X  10s 
joules)  and  this  will  gradually  replace  the  horsepower-hour 
because  it  is  based  on  a  system  of  international  units. 

18.  Heat  Energy  and  Electrical  Energy. — The  energy  required 
to  raise  the  temperature  of  1  Ib.  of  water  by  1°  F.  is  called 
the  British  Thermal  Unit  (B.T.U.)  and  is  equal  to  780  ft.  Ib. 
The   energy   required  to  raise  1  gm.  of  water  through  1°  C. 
is  called  the  gramme  calorie  and  is  equal  to  4.2  X  107  ergs  so 
that 

1  gm.  calorie    =  4.2  X  107  ergs 

=  4.2  watt-seconds  (joules). 

19.  Conversion  Factors. — Although  the  c.g.s.  system  of  units 
is    the    only  possible    international  system,  much   calculation 
work  is  still  carried  out  in  the  foot-pound-second  system.     The 
conversion  factors  given  below  help  to  simplify  the  work  of 
changing  from  one  system  to  another. 

C.g.s.  unit  Other  units 

Length  1  cm.  1  in.  =  2.54  cm. 

Mass  1  gm.  1  Ib.  =  453.6  gm. 

Time  1  sec. 

Force  1  dyne  1  gm.  =981  dynes 

1  Ib.     =  444,800  dynes 

=  453.6  gm. 
Work  or  energy  1  erg  =  1  dyne-cm.  1  joule  =  1  watt-sec. 

107  ergs 

1  ft.  Ib.  =   1.356  X  107  ergs 
1  kw.-hour  =  3600  X  103  joules 
1  gm.  calorie  =  4.2  joules 

1  Ib.  calorie  =    1900  joules 

Power  1  erg.  per  sec.  1  watt=   107  ergs  per  sec. 

1  kw.  =  1000  watts 
1  h.p.  =   550  ft.  Ib.  per  sec. 
=    746  watts 

20.  Problems  on  Work  and  Power. 

1.  A  hoist  raises  a  weight  of  2000  Ib.  through  a  distance  of  300  ft.  in  a 
time  of  1  min.  Find  the  work  done  and  the  power  expended. 

If  the  efficiency  of  the  hoist  is  75  per  cent,  and  that  of  the  motor  is  90  per 
cent,  find  the  horsepower  of  the  motor  and  also  the  current  taken  by  the 
motor  if  the  voltage  is  110. 

a.  Work  done  =  2000  X  300 

=  600,000  ft.  Ib. 


16          PRINCIPLES  OF  ELECTRICAL  ENGINEERING      [CHAP,  iv 

6.  Power  expended  =  600,000  ft.  Ib.  per  60  sec. 
=  10,000  ft.  Ib.  per  sec. 
10,000 

•  -w  •  18-2  h-p- 

18.2  X  746 

-.000-    =13'6kw- 

13.6 

c.  The  power  input  to  the  hoist  = =  18.1  kw  =  24.3  h.p. 

0.75 

18  1 

The  power  input  to  the  motor  =  — '—  =  20  kw. 

\j»  y          * 

d.  Since  watts  =  volts  X  amperes,  therefore  20  X  1000  =  110  X  amperes 
and  the  current  in  amperes  =  182. 

2.  An  electric  iron  takes  5  amp.  at  110  volts.    What  does  it  cost  to  operate 
this  iron  for  2  hours  if  the  cost  of  energy  is  6  cents  per  kw.-hour. 

The  rate  at  which  energy  is  used  =  110  X  5  =  550  watts 

=  0.55  kw. 

The  energy  used  in  2  hours  =  0.55  X  2  =  1.1  kw.-hour 
The    cost   of   this   energy  =  1.1     X  6  =  6.6  cents 

3.  A  32-candle  power,  110-volt  tungsten  lamp  requires  40  watts.     What  is 
the  current  taken  by  this  lamp  and  what  is  the  cost  of  energy  for  15  lamps 
burning  for  an  average  time  of  4  hours  if  the  cost  of  energy  is  5  cents  per 
kw.-hour? 

watts       40 

amperes  =  —  -  =  0.36  amp. 

volts        110 

power  =  40  X  15  =  600  watts 

=  0.6  kw. 

energy  used  =  0.6  X  4  =  2.4  kw. -hours 
cost  of  energy  =  2.4  X  5  =  12  cents. 

4.  An  electric  water  heater  has  an  efficiency  of  80  per  cent,  and  takes  3 
amp.  at  110  volts.     How  long  will  it  take  to  raise  1  pint  (1.25  Ib.)  of 
water  from  20°  C.  to  the  boiling  point  and  what  will  this  cost  when  the 
rate  is  5  cents  per  kw.-hour? 

energy  required  =  1.25  (100  -  20)  =  100  Ib.  calories 
=  100  X  1900  =  190,000  watt-sec. 

100 
energy  delivered  =  190,000  X  - 

80 

=  238,000  watt-sec. 
=  238  kw.-sec. 
=  0.066  kw. -hours 
cost  of  energy  =  0.066  X  5  =  0.33  cents 

Now  238,000  watt-sec,  are  supplied  at  the  rate  of  110  X  3  =  330  watts 
therefore  the  time  during  which  energy  must  be  applied 

238,000 


330 
12  min. 


=  720  sec. 


ART.  20]  WORK  AND  POWER  17 

5.  If  a  ton  (2000  Ib.)  of  coal  heats  a  house  for  a  month  what  would  it  cost 
to  give  exactly  the  same  heating  effect  electrically  if  the  cost  of  energy  is  3 
cents  per  kw.-hour  ? 

With  a  good  heating  system  1  Ib.  of  coal  burnt  on  the  grate  will  deliver 
8000  B.T.U.  or  4450  Ib.  calories  to  the  house. 

The  energy  required  per  month  therefore  =  4450  X  2000  Ib.  calories 

=  8,900,000  Ib.  calories. 
=  8,900,000  X  1900  watt-sec. 
8,900,000  X  1900 


3600  X  1000 


kw.-hcmrs 


=  4,700  kw.-hours 

the  cost  of  this  energy  =  4,700  X  3  =  14,100  cents 

=  141  dollars. 

The  reason  for  this  enormous  difference  in  the  cost  of  heating  by  the  two 
methods  is  that  the  efficiency  of  a  heating  system  is  about  60  per  cent, 
while  that  of  an  electric  generating  station  is  about  6  per  cent. ;  moreover 
the  cost  of  the  coal  required  per  kw.-hour  is  about  0.5  cents  or  1/6  of 
the  selling  price  of  the  electrical  energy. 


CHAPTER  V 


ELECTRIC  CIRCUITS  AND  RESISTANCE 

21.  The  flow  of  electricity  through  electric  circuits  is  similar 
in  many  ways  to  the  flow  of  water  through  hydraulic  circuits. 
This  may  be  seen  by  a  comparison  between  the  circuits  shown 
diagrammatically  in  Fig.  20. 


Pump 


FIG.  20. — Hydraulic  and  electric  circuits. 


To  maintain  a  steady  current 
of  w  gm.  of  water  per  sec. 
through  the  hydraulic  circuit 
and  to  raise  the  water  from  b  to 
a  through  a  difference  of  poten- 
tial of  h  cm.,  an  amount  of  power 
=  wh  gm.  cm.  per  sec.  must  be 
put  into  the  circuit  by  the 
pump. 

In  returning  from  a  to  b 
through  the  external  part  of  the 
circuit,  the  water  falls  through 
a  difference  of  potential  of  h  cm. 
and  supplies  an  amount  of 
power  =  wh  gm.  cm.  per  sec.  to 
drive  the  turbine  and  to  supply 
the  frictional  resistance  loss  in 
the  pipes. 


To  maintain  a  steady  elec- 
tric current  of  /  coulombs  per 
sec.  (amperes1)  through  the 
electric  circuit  and  to  raise  the 
electricity  through  a  difference 
of  potential  of  E  volts,  an 
amount  of  power  =  El  watts 
must  be  put  into  the  circuit  by 
the  electric  generator. 

In  returning  from  a  to  b 
through  the  external  part  of  the 
circuit,  the  electricity  falls 
through  a  difference  of  poten- 
tial of  E  volts  and  supplies  an 
amount  of  power  =  El  watts 
to  drive  the  motor  and  to 
supply  the  resistance  loss  in  the 
connecting  wires. 


XA  current  of  electricity  is  expressed  in  amperes;  there  is  no  corresponding 
unit  for  a  current  of  water  which  must  therefore  be  expressed  in  gm.  cm.  per 
sec.  The  quantity  of  electricity  which  passes  any  point  in  a  circuit  is  ex- 
pressed in  coulombs  where  1  coulomb  is  1  amp.-sec.  A  larger  unit  is  the 
ampere-hour. 

18 


AKT.  23]        ELECTRIC  CIRCUITS  AND  RESISTANCE 


19 


The  current  .of  water  (the  The  electric  current  (the 
quantity  passing  any  point  per  quantity  passing  any  point  per 
sec.)  is  the  same  at  all  points  sec.)  is  the  same  at  all  points 

in  the  circuit  since  the  circuit  is 

closed. 


in  the  circuit  since  the  circuit  is 
closed. 


22.  Ammeters  and  Voltmeters.— -The  current  in  a  circuit  may 
be  measured  by  means  of  an  instrument  such  as  that  described 
on  page  8,  connected  directly  in  the  circuit  as  shown  at  A, 
Fig.  20,  while  the  difference  of  potential  between  two  points 
may  be  measured  by  means  of  a  similar  instrument  connected 
directly  between  the  points  as  shown  at  B.  The  essential  differ- 
ence between  the  two  instruments  is  that  the  ammeter  must 
carry  the  total  current  in  the  circuit  with  only  a  small  difference 
of  potential  across  its  terminals  and  must  therefore  offer  a 
small  resistance  to  the  flow  of  current  through  it,  the  volt- 
meter on  the  other  hand  must  divert  only  a  small  portion  of 
the  current  from  the  circuit  and  must  therefore  offer  a  large 
resistance  to  the  flow  of  current  through  it. 


FIG.  21. — Hydraulic  and  electric  circuits. 


23.  Resistance  Circuits. — Consider  the  case  represented  dia- 
grammatically  in  Fig.  21  where  there  is  no  turbine  in  the  hydraulic 
circuit  nor  any  motor  in  the  electric  circuit. 


The  difference  of  potential  of 
h  cm.  maintained  by  the  pump 
is  used  up  in  forcing  w  gm.  of 
water  per  sec.  against  the  fric- 
tional  resistance  of  the  pipe,  and 

h  =  wr 

where  r  is  called  the  resistance 
of  the  pipe  circuit. 


The  difference  of  potential  of 
E  volts  maintained  by  the  elec- 
tric generator  is  used  up  in 
forcing  7  amperes  against  the 
resistance  of  the  wires,  and 

E  =  IR 

where  R  is  called  the  resistance 
of  the  electric  circuit  and  is  a 
constant  for  a  given  circuit. 


20  PRINCIPLES  OF  ELECTRICAL  ENGINEERING      [CHAP,  v 

This  resistance  increases  with  This  resistance  increases 
the  length  and  decreases  with  with  the  length  and  decreases 
the  cross  section  of  the  pipe.  with  the  cross  section  of  the 

wire,  .or 


R  =  k 

section 

24.  Ohm's  Law.  —  The  above  relation  E  =  IR  is  known  as 
Ohm's  law  and  the  unit  of  resistance,  called  the  ohm,  is  chosen 
of  such  a  value  that  a  circuit  with  a  resistance  of  1  ohm  will 
allow  1  amp.  to  flow  when  the  difference  of  potential  between 
the  ends  is  1  volt,  therefore 

volts  =  amperes  X  ohms 

If  for  example  the  current  in  the  heating  coil  of  a  110-  volt 
electric  iron  is  5  amp.,  then  the  resistance  of  this  coil  =  110/5 
=  22  ohms. 

25.  Specific  Resistance.  —  As  pointed  out  in  art.  23,  the  re- 
sistance of  a  wire  is  directly  proportional  to  its  length  and 
inversely  proportional  to  its  cross  section  or 


where  R  is  the  resistance  of  the  wire  in  ohms 

L  is  the  length  of  the  wire 

A  is  the  cross  section  of  the  wire 

k  is  a  constant  called  the  specific  resistance  and  depends 
on  the  material  and  on  the  units  chosen.  If  centimeter  units  are 
used  then  the  specific  resistance  is  the  resistance  of  a  piece  of 
the  material  1  cm.  long  and  1  sq.  cm.  in  cross  section  and  is 
expressed  in  ohms  per  cm.  cube. 

In  practice  the  unit  of  cross  section  is  generally  taken  as  the 
circular  mil  which  is  defined  as  the  cross  section  of  a  wire  1  mil 
(1/1000  in.)  in  diameter. 
Since  a  wire  1  mil  in  dia.  has  a  section  of  1  cir.  mil  a  wire  1 

inch  in  dia.  has  a  section  of  106  cir.  mils  and  a  wire  1  sq.  inch  in 

4 
section  has  a  section  of  -  106  cir.  mils. 

7T 

The  specific  resistance  of  copper  wire1  is  1.6  X  10~6  ohms  per 
cm.  cube  or  9.7  ohms  per  cir.  mil  foot  at  0°  C.;  the  specific 

1  For  values  of  specific  resistance  of  various  materials  see  Standard  Hand- 
book for  Electrical  Engineers. 


ART.  27]         ELECTRIC  CIRCUITS  AND  RESISTANCE  21 

resistance  of  cast   iron  is  80  X  10~6  ohms  per  cm.  cube  or  480 
ohms  per  cir.  mil  foot,  approx.,  at  0°  C. 

26.  Variation  of  Resistance  with  Temperature.  —  The  resist- 
ance of  most  materials  varies  with  the  temperature  and 

Rt  =  R0(l  +  at) 

where  Rt  is  the  resistance  at  t°  C. 
Ro  is  the  resistance  at  0°  C. 
t     is  the  temperature  of  the  material  in  deg.  C. 
a   is  called  the  temperature  coefficient  of  resistance. 

For  all  pure  metals  the  resistance  increases  with  the  tempera- 
ture and  a  is  approximately  equal  to  0,004.  The  resistance  of 
carbon  and  of  liquid  conductors  decreases  with  increase  of  tem- 
perature, while  the  resistance  of  special  alloys  such  as  manganin 
remains  approximately  constant  at  all  operating  temperatures. 

A  coil  has  1000  turns  of  copper  wire  with  a  cross  section  of  1288  cir.  mils 
and  a  length  of  mean  turn  of  15  in. 

a.  Find  the  resistance  of  the  coil  at  0°  C. 
6.  Find  the  resistance  of  the  coil  at  25°  C. 

c.  Find  the  current  that  will  flow  through  the  coil  at  25°  C.  and  110  volts. 

d.  After  current  has  passed  through  the  coil  for  some  time  it  is  found  that 
its  value  has  dropped  to  9  amp.,  find  the  average  temperature  of  the  coil 
under  these  conditions. 

a.  The  resistance  of  1  cir.  mil  foot  =  9.7  ohms  at  0°  C. 

9  7  X  1000  X  15 

The  resistance  of  the  coil  at  0°  C,  =  -  -  =  9.4  ohms. 

1288  X  12 

6.  The  resistance  of  the  coil  at  25°  C.  =9.4(1  +  0.004  X  25)  =  10.3  ohms. 

c.  The  current  =  ------  =  10.7  amp. 

10.  o 

d.  The  hot  resistance  of  the  coil  =  -  -  =  12.2  ohms  at  t°  C. 

9 

The  resistance  of  the  coil  also  =9.4  ohms  at  0°  C. 
Therefore  the  resistance  of  the  coil  at  t°  C.  =  9.4(1  +  0.0040 

=  12.2  ohms 

from  which  1  +  0.004*  =  12.2/9.4  =  1.3. 

and  t  =  0.3/0.004  =  75°  C. 

27.  Power  Expended  in  a  Resistance.  —  To  force  a  current  of 
7  amperes  through  a  circuit  which  has  a  resistance  of  R  ohms,  a 
voltage  E  =  IR  is  required  so  that 

the  power  expended  in  the  circuit  =  El  watts 


=  PR  watts. 
This  power  is  transformed  into  heat. 


22 


PRINCIPLES  OF  ELECTRICAL  ENGINEERING      [CHAP,  v 


In  the  electric  flat  iron  and  other  such  heating  apparatus  this 
heat  is  utilized,  the  heating  element  consisting  of  a  coil  of  high 
resistance  wire,  insulated  with  heat  resisting  insulation  such  as 
asbestos,  or  mica,  and  embedded  in  the-  iron. 

28.  Insulating  materials  are  materials  which  offer  a  very  large 
resistance  to  the  flow  of  electric  current  and  for  that  reason  they 
are  used  to  keep  the  current  in  its  proper  path.  In  a  transmission 
line,  current  is  prevented  from  passing  between  the  wires  by 
porcelain  insulators  attached  to  cross  arms  as  shown  in  Fig.  22. 
When  the  wires  are  placed  close  to  one  another,  as  in  house  wiring, 
they  are  covered  throughout  their  entire  length  with  insulating 
material  such  as  paper,  rubber  or  cotton;  the  electrical  resistance 
of  these  materials  is  greater  than  1010  ohms  per  cm.  cube. 


Porcelain 
Insulator 


/   V 


Cross  Arm 


FIG.  22. — Insulators  for  transmission  lines. 


29.  Dielectric  Strength  of  Insulating  Material. — If  a  sheet  of 
insulating  material  is  placed  between  two  terminals  and  the 
voltage  between  the  terminals  is  gradually  raised  the  material 
will  finally  break  down  and  a  hole  be  burnt   through   it,   the 
material  is  then  said  to  be  punctured,  and  a  large  current  will  flow 
through  the  puncture  if  the  voltage  is  maintained.     The  property 
of  an  insulating  material  by  virtue  of  which  it  resists  breakdown 
is  called  its  dielectric  strength. 

30.  Series  and  Parallel  Circuits. — If  several  conductors  are 
connected  in  series  as  shown  in  Fig.  23,  then  the  current  is  the 


ART.  31]        ELECTRIC  CIRCUITS  AND  RESISTANCE 


23 


same  in  each  conductor  while  the  total  voltage  is  the  sum  of  the 
voltages  across  the  different  parts  of  the  circuit  so  that 


E  =  EI  -f-  EZ  H~ 


-f- 


If  several  conductors  are  connected  in  parallel  as  shown  in  Fig. 
24,  then  the  voltage  across  each  conductor  is  the  same  while  the 
total  current  is  the  sum  of  the  currents  in  the  different  paths  so 
that 

/    .=    Il   +   /2   +   /8   +   1  4 

' 


nr*Vvwwj- rA/WVVT— 

£7  ^ 

L_S4_J       U ^->| 


K4        I  Rs 

FIG.  23. — Series  circuit. 


FIG.  24. — Parallel  circuit. 


Four  coils  having  resistances  of  3,  5,  10  and  12  ohms  respectively  are 
connected  in  series  across  120  volts,  find  the  current  in  the  circuit  and  the 
voltage  drop  across  each  coil. 

120  =  7(3  +  5  +  10  +  12) 

=  307 
therefore         7=4  amp  . 

Ej_  =  4  X  3  =  12  volts 
E2  =  4  X  5  =  20  volts 
E3  =  4  X  10  =  40  volts 
#4  =  4  X  12  =  48  volts 

If  these  coils  are  now  connected  in  parallel  across  120  volts,  find  the  current 
in  each  coil  and  also  the  total  current. 

71  =  120/3     =  40  amp. 

72  =  120/5     =  24  amp. 

73  =  120/10  =  12  amp. 

74  =  120/12  =  10  amp. 
total  current     7  =86  amp. 

31.  Voltage  Drop  in  a  Transmission  Line. — When  electric 
energy  is  transmitted  from  one  point  to  another  over  wires,  a 


24  PRINCIPLES  OF  ELECTRICAL  ENGINEERING     [CHAP.V 

voltage,  called  the  drop  in  the  line,  is  required  to  force  the  cur- 
rent through  the  wires.  This  voltage  =  IR  where  R  is  the  total 
resistance  of  the  connecting  wires  and  7  is  the  current  flowing, 
so  that  if  Eg,  Fig.  25,  is  the  voltage  at  the  generating  station,  and 
Er  is  the  voltage  applied  to  the  load  in  the  receiving  station, 
then  Ea  =  Er  +  IR. 


FIG.  25. 

The  power  to  be  delivered  at  the  end  of  a  2  mile  line  is  30  kw.  If  the 
receiver  voltage  is  600,  find  the  size  of  wire  required  to  limit  the  voltage  drop 
in  the  line  to  5  per  cent.,  find  also  the  power  loss  in  the  line. 

30  X  1000 

Current  in  line  =  —  -  =  50  amp. 

600 

The  voltage  drop  in  the  line  =  5  per  cent,  of  600  =  30  volts 
The  resistance  of  the  wijre  in  the  line  =  30/50  =  0.6  ohms 
The  resistance  of  copper  =  9.7  ohms  per  cir.  mil  foot  at  0°  C. 
=  9.7  (1  +  0.004  X  25) 
=  10.6  ohms  per  cir.  mil  foot  at  25°  C. 
The  resistance  of  2  miles  of  line  or  4  miles  of  wire 

10.6  X4  X5280 

-  =  0.6  ohms 
cir.  mils 

From  which  cir.  mils  =  370,000 

The  loss  in  the  line  =  30  volts  X  50  amp. 

=  1500  watts 

=  5  per  cent,  of  theNpower  delivered. 


CHAPTER  VI 
RHEOSTATS  AND  RESISTORS 

32.  Rheostats. — A  rheostat  is  an  adjustable  resistance  of 
such  a  form  that  it  can  be  conveniently  used.  In  the  rheostat 
shown  in  Figs.  26  and  27,  the  resistance  ab  is  tapped  at  eight 


FIG.  26. 


SUTE 


FIG.  27. — Sliding  contact  type  of  rheostat. 

points  which  are  connected  to  contact  studs  s  over  which  the 
handle  H  is  free  to  move. 

Such  a  rheostat  is  used  to  control  the  current  in  a  circuit. 

25 


26 


PRINCIPLES  OF  ELECTRICAL  ENGINEERING      [CHAP,  vi 


When  the  handle  is  in  the  position  shown  in  Fig.  26,  all  the  re- 
sistance ab  is  in  the  circuit  and  the  current  /  has  its  minimum 
value.  When  the  handle  is  in  the  position  Hi,  current  flows 
through  the  path  TcdebQ  so  that  the  resistance  between  a  and 
e  has  been  cut  out.  As  the  handle  is  moved  further  over,  the 
resistance  in  the  circuit  is  further  decreased  until  finally,  when 
the  handle  is  in  the  position  Hz,  the  resistance  is  all  cut  out  and 
the  current  in  the  circuit  has  its  maximum  value. 

33.  Resistors. — For  economy  in  manufacture,  resistances  such 
as  ab,   Fig.   26,   are  generally  built   up  of  standard  resistance 


Slate 


FIG.  28. 


FIG.  30. 


FIG.  29. 

Resistance  units. 


FIG.  31. 


units  called  resistors,  which  may  be  connected  in  series  or  in 
parallel  as  desired.  Different  types  of  resistance  units  are  shown 
in  Figs.  28  to  33. 

The  unit  shown  in  Fig.  28  consists  of  a  length  of  wire  wound 
on  an  iron  tube,  from  which  it  is  insulated  by  fireproof  insulation 
such  as  asbestos. 

In  Fig.  29  a  similar  unit  is  shown  which  consists  of  a  length  of 
wire  wound  in  a  spiral  groove  cut  on  the  surface  of  a  tube  of 
porcelain  or  some  other  such  material,  adjoining  turns  of  the 
wire  being  thereby  separated  from  one  another. 

Such  units  are  mounted  in  frames  as  shown  in  Fig.  27.  They 
are  always  placed  vertically  so  that  air  can  circulate  freely 
through  the  tubes  and  over  the  surface  of  the  resistance  wire 
and  thereby  keep  the  temperature  of  the  rheostat  within 
reasonble  limits. 

For  carrying   comparatively    small    currents,   round  wire  is 


ART.  35] 


RHEOSTATS  AND  RESISTORS 


27 


suitable;  strip  metal  is  preferred  for  larger  currents,  as  it  gives 
a  larger  surface  for  a  given  section.  An  excellent  type  of  con- 
struction is  shown  in  Fig.  32  where  the  resistance  unit  consists 
of  a  length  of  resistance  strip  metal  wound  on  a  frame  consisting 
of  an  iron  plate  A  insulated  at  the  edges  with  porcelain  sup- 
porting pieces  B.  These  units  may  be  mounted  on  iron  rods 
which  pass  through  the  holes  C. 

34.  Heater  Units. — Fig.  30  shows  the  external  appearance 
of  a  type  of  resistor  which  is  largely  used  for  electric  irons 
and  other  such  heating  appliances.  It  is  constructed  of  re- 
sistance strip  wound  in  the  form  of  a  helix  and  placed  in  a  metal 
tube  which  is  lined  with  mica,  the  tube  is  then  packed  with  fire- 


FIG.  32. — Resistance  unit. 

proof  cement  to  insulate  adjacent  turns  from  one  another,  and 
the  open  end  of  the  tube  is  closed  with  a  cement  plug  through 
which  the  leading  in  wires  are  brought. 

Another  type  of  heater  unit  is  shown  in  Fig.  31  and  consists 
of  a  length  of  resistance  wire  wound  into  a  helix  of  small  diameter, 
which  helix  is  then  coiled  into  a  flat  spiral  and  mounted  in  a 
frame  with  mica  between  the  convolutions.  This  unit  is  held 
against  a  layer  of  quartz  grains  which  are  embedded  in  enamel  on 
the  bottom  of  the  heater. 

35.  Cast-iron  Grid  Resistance. — When  large  currents  have  to  be 
controlled,  the  necessary  cross  section  to  carry  the  current  and  the 
necessary  radiating  surface  to  dissipate  the  heat  are[best  obtained 
,by  the  use  of  zig-zag  units  of  the  shape  shown  in  Fig.  33.  For 
small  rheostats,  these  zig-zag  pieces  may  be  punched  out  of  sheet 
metal,  but  for  larger  sizes  they  are  generally  of  cast  iron  as 
shown  in  Fig.  34. 

The  method  or  assembling  these  castings  is  shown  in  Fig.  35 
which  is  a  plan  of  a  rheostat  similar  to  that  in  Fig.  34.  The  units 


28 


PRINCIPLES  OF  ELECTRICAL  ENGINEERING      [CHAP,  vi 


A  are  mounted  on  iron  rods  B  which  are  insulated  throughout  their 
entire  length  by  mica  or  asbestos  tubes  C.     The  individual  units 


FIG.  33.— Zig-zag  resistance  unit. 


FIG.  34. — Cast-iron  grid  resistance. 


Iton  Rod 


D   Metal  Washer 

F  Insulating  Washer 


Mica  Tube 


FIG.  35. — Flow  of  current  in 
a  grid  resistance. 


FIG.  36. — Carbon  pile  rheostat. 


are  separated  from  one  another  by  washers  which  are  either  of 
metal  as  at  D  and  E  or  of  insulating  materials  as  at  F,  depending 


ART.  37] 


RHEOSTATS  AND  RESISTANCE 


29 


on  the  direction  in  which  it  is  desired  to  make  the  current  flow. 
The  four  metal  washers  E  act  as  terminals  from  which  leads 
can  be  taken  to  the  contacts  on  the  control  faceplate. 

36.  Carbon  Pile  Rheostat. — An  entirely  different  type  of  rheo- 
stat is  shown  in  Fig.  36  and  consists  of  a  column  of  graphite  discs 
A,  enclosed  in  a  steel  tube  B  which  is  lined  with  fireproof  insula- 
tion such  as  asbestos.  The  resistance  of  such  a  pile  decreases  as 
the  mechanical  pressure  between  the  ends  increases,  because  the 
contact  between  adjacent  discs  improves.  In  the  type  of  rheo- 
stat shown  in  Fig.  36  the  pressure  is  applied  by  turning  the  hand- 
wheel  D  and  is  communicated  to  the  carbon  pile  through  the 
plungers  E.  The  two  units  shown  may  be  connected  in  series  or 
in  parallel  as  desired,  and  the  resistance  of  such  a  rheostat  can  be 
changed  gradually  through  a  total  range  of  about  100  to  1. 


FIG.  37. — Liquid  rheostat. 

37.  Liquid  Rheostats. — Such  a  rheostat  is  shown  in  Fig.  37 
and  consists  of  a  cast-iron  trough  A  which  contains  a  solution  of 
caustic  soda  or  some  similar  material  which  does  not  attack  iron, 
and  an  iron  plate  B  which  is  insulated  from  the  tank  as  shown  at  E 
and  which  dips  into  the  liquid.  Between  the  terminals  T\  and  T2 
therefore  there  is  the  resistance  of  the  path  through  the  liquid 
between  A  and  B,  and  the  section  of  this  path  can  be  increased  or 
decreased  by  lowering  or  raising  the  plate  B.  The  resistance  may 
be  finally  short  circuited  by  lowering  B  far  enough  to  allow  the 
contact  H  to  close,  then  current  can  pass  direct  from  T\  to  T2 
without  passing  through  the  liquid. 

Another  type  of  liquid  rheostat  is  shown  in  Figs.  38  and  39. 


30 


PRINCIPLES  OF  ELECTRICAL  ENGINEERING     [CHAP,  vi 


In  this  case  the  plates  are  fixed  but  the  level  of  the  liquid  is 
varied.  The  pump  D  sends  a  continuous  stream  of  liquid  from 
the  cooling  chamber  A  through  the  resistance  chamber  B,  and 
the  level  of  the  liquid  in  this  latter-  chamber  may  be  raised 
or  lowered  by  a  weir  C. 


IjJ  Pipe  Tap 


FIG.  38. 


FIG.  39. — Liquid  rheostat. 

The  liquid  is  cooled  in  the  lower  chamber  by  water  which  flows 
through  cooling  pipes.  This  cooling  chamber  sometimes  takes  the 
form  of  a  concrete  tank  made  large  enough  to  allow  the  rheostat 
to  be  self  cooling. 


ART.  38]  RHEOSTATS  AND  RESISTORS  31 

Liquid  rheostats  are  largely  used  in  making  load  acceptance 
tests  on  generators.  Two  electrodes  in  a  barrel  of  water  in  which 
a  handful  of  common  salt  has  been  dissolved  will  dissipate  about  5 
kw.  if  the  water  is  stationary.  The  type  of  temporary  rheostat 
most  generally  used  however  consists  of  a  bank  of  cast-iron  grids 
mounted  in  a  wooden  frame  and  placed  in  running  water,  the  grids 
will  carry  about  four  times  as  much  current  under  these  conditions 
as  when  air  cooled.1 

38.  The  size  of  a  rheostat  depends  principally  on  the  amount 
of  power  which  it  is  required  to  dissipate.  If  two  rheostats  have 
to  dissipate  the  same  amount  of  power  but  one  has  only  half  as 
much  current  flawing  as  the  other  then,  since  the  loss  in  the 
rheostat  =  PR  watts,  the  former  rheostat  must  have  four  times 
the  resistance  of  the  latter,  that  is  the  wire  must  have  half  the 
section  and  twice  the  length,  but  the  weight  of  wire  and  the  space 
occupied  by  the  rheostat  will  be  approximately  the  same  in  each 
case. 

1  For  design  data  on  such  temporary  rheostats  see  the  Standard  Handbook 
for  Electrical  Engineers. 


CHAPTER  VII 

MAGNETIC  CIRCUITS  AND  MAGNETIC  PROPERTIES 

OF  IRON 

39.  Magnetic  Field  due  to  a  Solenoid. — A  solenoid  is  a  coil  of 
wire  wound  in  the  form  of  a  helix  as  shown  in  Fig.  8,  page  5. 
When  an  electric  current  is  passed  through  such  a  coil  it  acts  as 
an  electromagnet  and  the  direction  of  the  magnetic  field  may  be 
found  by  the  corkscrew  law,  page  5. 


A  Sq  Cm. 


T    Turns 
i 

FIG.  40. — Closed  solenoid. 

The  solenoid  in  Fig.  40  has  T  turns  wound  on  a  cardboard  spool  and  is 
bent  to  form  an  annular  ring.  A  current  of  i  c.g.s.  units  flowing  through 
these  T  turns  produces  a  magnetic  field  of  intensity  3C  which  field  can 
therefore  be  represented  by  3C  lines  of  force. 

If  a  unit  pole  n  be  moved  once  round  the  magnetic  circuit  through  a  dis- 
tance of  2-n-r  centimeters  in  a  time  of  t  seconds  then,  since  the  force  on  this 
pole  due  to  the  electromagnet  is  3C  dynes,  the  work  done  in  moving  the 
pole  =  JC  X  2irr  ergs.  But  a  unit  pole  has  4x  lines  of  force,  see  page  3, 
and  while  this  pole  is  moved  once  around  the  magnetic  circuit  these  lines 
cut  the  T  turns  of  the  coil  in  a  time  of  t  seconds  and  generate  in  the  coil  an 
e.m.f.  e,  which  in  c.g.s.  units  =  4irT/t,  the  number  of  lines  cut  per  second. 
The  coil  therefore  acts  as  a  generator  and  supplies  an  amount  of  power  =  ei 
ergs  per  second  so  long  as  the  unit  pole  is  moving,  that  is  for  a  time  of  t 
seconds.  This  power  must  be  obtained  at  the  expense  of  the  power  ex- 
pended in  keeping  the  unit  pole  moving  so  that 

32 


ART.  41]  MAGNETIC  PROPERTIES  OF  IRON  33 

3C  X  27r  r  =  eit  ergs 


Ti 
therefore  5C  =  4w  -j-  where  i  is  in  c.g.s.  units  and  L  =  2?rr 

47T    T/ 

=  TQ    T.  where  /  is  in  amperes. 

In  a  magnetic  circuit  such  as  that  shown  in  Fig.  40  the  field 
intensity  3C  is  given  by  the  formula 

47T    TI 

~~  10  L 

where  I  is  the  current  in  amperes 

T  is  the  number  of  turns  of  the  solenoid 

TI  is  called  the  ampere-turns 

L  is  the  length  of  the  magnetic  circuit  in  cm.  =  2-nr  in  the 

above  case 

5C  is  the  field  intensity  in  the  magnetic  circuit  and  is  also 
the  flux  density  or  the  number  of  lines  of  force  per  sq. 
cm.  of  solenoid  cross  section. 
The  total  magnetic  flux  threading  the  magnetic  circuit  is 

0  =  3C.A 

47r77 
=  10  LA 

where  A  is  the  cross  section  of  the  solenoid  in  sq.  cm. 

40.  Permeability.  —  If  the  solenoid  is  wound  on  a  core  of  mag- 
netic material  such  as  iron  or  steel  it  is  found  that  for  the  same 
number  of  exciting  ampere-turns  a  much  larger  magnetic  flux  is 
produced  and  that 

'(B,  the  flux  density  =  ^  -j^-  M  lines  per  sq.  cm. 

<f>,  the  magnetic  flux  =  y^  -j-  A  ju  lines  of  magnetic  flux. 

where  ju  is  a  quantity  called  the  permeability  of  the  material  and 
is  equal  to  unity  for  air  and  is  greater  than  unity  for  magnetic 
materials  such  as  iron  and  steel. 

41.  Reluctance  of  a  Magnetic  Circuit.  —  The  above  general  law 
for  the  magnetic  circuit  may  be  expressed  in  slightly  different  form 
namely 

,       4^  TJ  v  ^M 

0  =  id  TI  x  ~L 


34        PRINCIPLES  OF  ELECTRICAL  ENGINEERING       [CHAP,  vn 

m.m.f. 

(R 
or  m.m.f.  =  </>(R 

where  m.m.f.  called    the  magnetomotive   force,   is    that    which 

4rr 
produces  the  magnetic  flux  and  =  ^TI  ampere-turns 

$  is  the  number  of  lines  of  magnetic  flux  in  the  magnetic  circuit 
(R  called  the  reluctance  of  the  magnetic  circuit  =     - ,- 

From  its  similarity  to  the  law  for  the  electric  circuit,  namely 
e.m.f.  —  IR,  the  above  law  is  sometimes  called  Ohm's  law  for 
the  magnetic  circuit. 

Since  the  permeability  of  iron  is  much  greater  than  that  of  air, 
the  reluctance  of  an  iron  path  is  much  lower  than  that  of  an  air 
path  of  the  same  dimensions. 


I8xio 


10       20       30       40       50       60       70       80       90     100 
Ainpure  Turns  per  Cm. 

FIG.  41. — Magnetization  curves. 

42.  Magnetization  Curves. — The  magnetic  properties  of  iron 
and  steel  are  generally  shown  by  means  of  magnetization  curves 
such  as  those  in  Fig.  41;  the  data  from  which  these  curves  are 
plotted  is  determined  in  the  following  way. 

Test  pieces  of  iron  are  made  in  the  form  of  an  annular  ring 
with  a  cross  section  of  A  sq.  cm.  and  a  mean  length  of  magnetic 
path  of  L  cm.  These  rings  are  then  wound  uniformly  with  T 
turns  of  wire  as  in  Fig.  40  and  the  flux  <j>  is  measured  for  different 
values  of  the  current  /  by  means  of  special  instruments. 


ART.  43] 


MAGNETIC  PROPERTIES  OF  IRON 


35 


The  value  of  0/A,  the  flux  density,  is  then  plotted  against 
corresponding  values  of  TI/L,  the  ampere-turns  per  unit  length 
of  magnetic  path,  as  shown  in  Fig.  41,  to  give  what  is  called  the 
magnetization  curve  of  the  material. 

^r  may  then  be  determined. 

When  this  value  is  plotted  against  flux  density,  as  in  Fig.  42, 
it  may  be  seen  that,  once  a  particular  density  has  been  reached, 
the  permeability  decreases  rapidly  with  increase  of  flux  density. 
Permeability  curves  are  seldom  used  in  practice,  it  is  found  to 
be  more  convenient  to  work  with  magnetization  curves  such  as 


The  permeability  JJL  =  -mfr- 


2000 
1800 
1600 
1400 
1200 
1000 
800 
600 
400 
200 

X 

\ 

\ 

\ 

\ 

:> 
& 

\% 

j2z 

\ 

Y 

—  ==; 

\ 

\ 

^ 

^ 

\ 

\ 

^v. 

*  — 

*••  • 

\ 

2         4         6         8        10       12       14        16       18xl03 
Flux  Density  in  Lines  per  Sq.  Cm. 

FIG.  42. — Permeability  curves/ 

those  shown  in  Fig.  41.     An  example  of  the  use  of  such  curves 
is  given  on  page  44. 

43.  Residual  Magnetism. — If,  after  a  piece  of  iron  has  been 
magnetized  by  means  of  an  exciting  coil,  the  exciting  current  is 
reduced  to  zero,  it  will  be  found  that  the  magnetism  has  not  be- 
come zero  but  that  some  of  it,  called  the  residual  magnetism, 
remains.  If  the  iron  is  soft  and  annealed,  this  residual  magnetism 
will  be  of  negligible  amount  and  the  last  traces  of  it  may  be  made 
to  disappear  if  the  iron  is  subjected  to  vibration.  If  hard  tool 
steel  is  used  the  residual  magnetic  field  will  be  strong  and  the 
residual  magnetism  can  be  removed  only  with  difficulty  so  that 
permanent  magnets  are  generally  made  of  this  material. 


36        PRINCIPLES  OF  ELECTRICAL  ENGINEERING       [CHAP.VII 

44.  Molecular  Theory  of  Magnetism. — To  account  for  the 
peculiar  magnetic  behavior  of  iron,  Ewing  suggested  that  mole- 
cules of  iron  are  natural  magnets  each  with  its  own  north  and 
south  pole.  When  the  iron  does  not  exhibit  magnetic  properties 
then  the  molecular  magnets  are  arranged  in  groups  as  shown  in 
diagram  A,  Fig.  43,  and  their  magnetic  effects  neutralize  each 
other. 

If  the  iron  is  placed  in  a  strong  magnetic  field  the  molecular 
magnets  will  turn  and  point  in  the  direction  of  the  field  as  shown 
in  diagram  B,  Fig.  43. 

If  a  piece  of  iron  is  placed  in  an  exciting  coil,  a  small  current  in 
this  coil  will  turn  these  molecular  magnets  which  are  not  strongly 
held  together  and  will  line  them  up  in  the  direction  of  the  mag- 


A    not  Magnetized  B   Magnetized 

FIG.  43. — Arrangement  of  the  molecules  of  an  iron  bar. 

netizing  force,  these  magnets  will  then  add  their  own  magnetic 
flux  to  that  which  the  coil  would  produce  if  no  iron  were  present. 
As  the  exciting  current  is  increased,  more  of  these  magnets  are 
lined  up  until,  when  the  point  B  has  been  reached  on  the  curve 
in  Fig.  41  all  but  the  most  rigid  of  the  molecular  magnets  have 
been  lined  up  and  the  magnetic  flux  can  then  increase  but  little 
even  for  a  large  increase  in  the  excitation. 

When  the  exciting  current  is  reduced  to  zero  and  the  magnetiz- 
ing force  thereby  removed,  the  molecular  magnets  reform  into 
groups  but,  on  account  of  molecular  friction,  they  do  not  return 
quite  to  their  original  position  but  have  a  slight  permanent  dis- 
placement in  the  direction  in  which  they  have  been  magnetized 
and  this  accounts  for  the  residual  magnetism. 

45.  Hysteresis. — There  is  another  phenomenon  in  connection 
with  the  magnetization  of  iron  which  can"  readily  be  explained 
by  the  molecular  magnet  theory,  namely,  that  if  the  magnetism 
of  a  piece  of  iron  is  reversed  rapidly  the  iron  becomes  hot.  What 
is  called  hysteresis  energy  has  to  be  expended  in  overcoming  the 
molecular  friction  of  the  magnets  and  this  appears  in  the  form  of 
heat. 


CHAPTER  VIII 
SOLENOIDS  AND  ELECTROMAGNETS 

46.  Pull  of  Solenoids. — A  solenoid  is  a  conductor  wound  in 
the  form  of  a  helix.  When  an  electric  current  is  passed  round  a 
solenoid  a  magnetic  field  is  produced,  the  direction  of  which  may 
be  determined  by  the  corkscrew  law,  page  5.  This  field  may 
be  represented  by  lines  of  force  as  shown  in  diagram  A,  Fig.  44. 


FIG.  44. — Action  of  a  solenoid. 

If  long  bar  magnets  are  placed  in  the  solenoid  field  as  shown  in 
diagram  B,  Fig.  44,  then  the  n  pole  of  magnet  x  will  tend  to  move 
in  the  direction  of  the  lines  of  force,  see  page  2,  and  be  pulled 
into  the  solenoid,  while  the  s  pole  of  magnet  y  will  tend  to  move 
in  a  direction  opposite  to  that  of  the  lines  of  force  so  that  it  also 
tends  to  move  into  the  solenoid. 

If  the  current  in  the  solenoid  is  reversed,  the  magnetic  field  of 
the  solenoid  will  reverse  and  the  magnets  x  and  y  will  be  repelled. 

37 


38       PRINCIPLES  OF  ELECTRICAL  ENGINEERING       [CHAP,  vm 


If  as  in  diagram  C,  Fig.  44,  soft  iron  plungers  are  used  instead 
of  bar  magnets,  then  the  lines  of  force  produced  by  the  solenoid 
will  pass  through  the  plungers  and  cause  magnetic  poles  to  be 
induced;  north  poles  will  be  formed  where  the  lines  of  force  leave 
the  iron  and  south  poles  where  they  enter,  see  page  3.  The 
induced  polarity  of  the  plungers  shown  in  diagram  C  is  the  same 
as  the  polarity  of  the  bar  magnets  in  diagram  B  so  that  the 
plungers  are  pulled  into  the  solenoid. 

If  the  current  in  the  solenoid  is  now  reversed,  the  magnetic 
field  of  the  solenoid  will  reverse  but,  since  the  induced  polarity 
of  the  plungers  will  also  reverse,  the  direction  of  the  pull  on  the 
plungers  will  be  unchanged. 

47.  Electric  Hammer. — The  two  types  of  electric  hammer 
shown  diagrammatically  in  Figs.  45  and  46  illustrate  the  action 
of  a  solenoid  with  a  magnet  plunger  and  with  a  soft  iron  plunger 
respectively. 


FIG.  45.  FIG.  46. 

Diagrammatic  representation  of  electric  hammers. 

In  Fig.  45,  current  passed  through  coil  C  makes  the  iron  plunger 
into  a  magnet  with  the  polarity  as  shown.  If  now  a  current  /  is 
passed  through  coils  A  and  B  in  the  direction  indicated  by  the 
arrows,  then  the  plunger  p  will  be  attracted  by  A  and  repelled 
by  B  and  will  move  toward  the  left.  If  this  current  /  is  now  re- 
versed, the  plunger  will  move  in  the  opposite  direction,  so  that, 
by  continually  reversing  the  current  that  flows  through  A  and  B, 
the  plunger  p  may  be  made  to  reciprocate. 

Another  type  of  hammer  is  shown  in  Fig.  46.  The  soft  iron 
plunger  is  pulled  into  the  position  shown  when  coil  A  is  excited, 
while  if  coil  B  is  excited  the  plunger  is  pulled  into  this  latter 
coil.  By  alternately  exciting  the  two  coils,  the  plunger  may  be 
made  to  reciprocate. 

48.  Variation  of  the  Pull  of  a  Solenoid. — When  the  plunger  is 
in  the  position  shown  in  A,  Fig.  47,  the  reluctance  of  the  magnetic 


ART.  49] 


SOLENOIDS  AND  ELECTROMAGNETS 


39 


circuit  is  large  since  the  path  of  the  lines  of  flux  is  nearly  all 
through  air,  so  that  the  magnetic  field  and  the  plunger  poles  are 
both  weak.  As  the  plunger  moves  toward  F,  the  reluctance  of 
the  magnetic  circuit  decreases  because  the  amount  of  iron  in  the 
magnetic  path  is  increasing,  so  that  the  magnetic  field  and  the 
plunger  poles  become  stronger. 

With  further  motion  of  the  plunger  in  the  same  direction,  the 
reluctance  of  the  magnetic  circuit  continues  to  decrease  and  the 


Distance   x 


FIG.  47. — Pull  of  a  solenoid. 

strengths  of  the  magnetic  field  and  of  the  plunger  poles  to 
increase,  but  the  induced  south  pole  of  the  plunger  now  begins 
to  come  under  the  influence  of  the  solenoid  field  and  is  repelled 
so  that,  although  the  north  pole  is  still  attracted,  the  resultant 
pull  decreases  and  finally  becomes  zero  when  the  plunger  is  in  the 
position  shown  in  diagram  B;  the  reluctance  of  the  magnetic 
circuit  has  then  its  minimum  value. 

The  pull  on  the  plunger  varies  with  its  position  as  shown  in 
diagram  C;  over  a  considerable  range  the  pull  is  constant. 

49.  Circuit  Breaker. — The  variation  in  the  pu,ll  of  a  solenoid  is 
taken  advantage  of  in  the  type  of  circuit  breaker  shown  dia- 
grammatically  in  Fig.  48.  Such  a  circuit  breaker,  consists  of  the 


40       PRINCIPLES  OF  ELECTRICAL  ENGINEERING      [CHAP,  vin 

switch  C  closed  against  the  force  of  the  spring  S  and  held  closed 
by  the  latch  d.  This  latch  is  released  by  the  plunger  p  which  is 
lifted  when  the  line  current  passing  round  the  solenoid  M  reaches 
a  predetermined  value,  the  spring  S  then  forces  the  switch  open. 
If  the  plunger  p  is  moved  further  into  the  solenoid  by  means  of 
the  adjusting  screw  a  then  the  current  required  to  lift  this  plunger 
will  be  decreased,  by  this  means  the  circuit  breaker  can  be 
adjusted  to  open  with  different  currents. 


Iron 


.—  -^Solenoid  M 


Slate  Base 


FIG.  48. — Automatic  circuit 
breaker. 


FIG.  49. — Electromagnetic  motor. 


50.  Laws  of  Magnetic  Pull. — The  law  of  inverse  squares,  art. 
2,  page  1,  applies  only  to  imaginary  point  magnets;  in  prac- 
tical work  the  following  laws  are  applied. 

The  force  on  a  piece  of  iron  in  a  magnetic  field  in  air  tends  to 
move  the  iron  in  such  a  direction  as  to  reduce  the  reluctance  of 
the  magnetic  circuit. 

The  magnitude  of  this  force  at  any  point  is  proportional  to  the 
space  rate  of  change  of  the  magnetic  flux  as  the  iron  passes  the 
given  point. 

An  interesting  application  of  this  rule  is  shown  diagrammatic- 
ally  in  Fig.  49.  The  lines  of  force  due  to  the  coils  A  and  B  pass 
through  the  magnetic  circuit  as  shown  by  the  arrows  and  the 
pivoted  piece  of  iron  p  tends  to  move  until  the  reluctance  of  the 
magnetic  path  is  a  minimum,  that  is,  until  the  air  gaps  between 
n  and  s  have  their  minimum  value  and  p  is  pointing  in  the 
direction  ab.  If  the  shape  of  the  curved  parts  from  a  to  b  is  such 
that  the  magnetic  flux  0  increases  uniformly  with  the  angle  turned 


ART.  51] 


SOLENOIDS  AND  ELECTROMAGNETS 


41 


through  by  p  then  the  turning  force,  being  proportional  to  the 
space  rate  of  change  of  flux,  will  be  constant  over  the  whole 
range  of  motion. 

The  above  principle  is  frequently  used  in  toy  electromagnetic 
motors,  provision  being  made  for  cutting  off  the  current  in  the 
exciting  coils  when  p  approaches  close  to  the  position  ab  and  for 
switching  the  current  on  again  when  this  point  is  passed. 

51.  Solenoids  with  Long  and  with  Short  Plungers. — When  the 
plunger  is  of  the  same  length  as  the  solenoid,  the  pull  becomes  zero 
when  the  plunger  is  in  the  position  shown  in  diagram  B,  Fig. 
47,  the  position  of  minimum  reluctance. 


Distance  X 

FIG.  50. — Pull  of  a  solenoid. 

When  the  plunger  is  longer  than  the  solenoid,  as  is  generally 
the  case  in  practice,  the  reluctance  does  not  become  a  mini- 
mum until  the  plunger  projects  equally  from  both  ends  as 
shown  in  diagram  B,  Fig.  50,  so  that  the  range  of  the  solenoid 
is  increased,  as  may  be  seen  by  a  comparison  between  the  curves 
in  Fig.  47  and  Fig.  50. 

52.  Ironclad  Solenoids. — In  order  to  reduce  the  reluctance  of 
the  return  part  of  the  magnetic  circuit  and  at  the  same  time  to 
protect  the  windings,  the  ironclad  construction  shown  in  Fig. 
51  is  used.  When  the  plunger  is  in  the  position  shown  in  diagram 
A,  the  reluctance  of  the  magnetic  circuit  is  nearly  all  in  the  air 
path  ab.  As  the  plunger  moves  toward  6,  the  flux  increases  and 
changes  very  rapidly  toward  the  end  of  the  stroke  so  that,  while 


42       PRINCIPLES  OF  ELECTRICAL  ENGINEERING      [CHAP.VIII 

the  average  pull  is  not  much  higher  than  that  of  the  same  solenoid 
with  an  air  return  path,  a  large  pull  over  a  short  distance  is  ob- 


.Stopped  Jroncjad  Type 

PIG.  51. — Types  of  ironclad  electromagnet. 

tained  at  the  end  of  the  stroke  as  shown  in  curve  B,  Fig.  52. * 
If  a  hole  for  the  plunger  be  bored  through  the  iron  cover  as 


10     n     12 


FIG.  52. — Pull  of  electromagnets. 

shown  in  diagram  B,  Fig.  51,  then  there  is  no  sudden  jar  at  the  end 
of  the  stroke  but  rather  a  cushion  effect;  the  large  increase  of  pull 

*Taken  from  an  article  by   Underbill,  Electrical  World  and   Engineer, 
Vol.  45,  p.  934  (1905.) 


ART.  53] 


SOLENOIDS  AND  ELECTROMAGNETS 


43 


at  the  end  of  the  stroke  is  lost  however,  although  this  is  seldom 
a  disadvantage. 

Fig.  53  shows  a  series  of  test  curves  on  ironclad  magnets  of 
the  cushion  type  and  will  give  the  reader  some  idea  of  the  magni- 
tude and  range  of  pull  that  can  be  obtained. 


\\l\\\\    MINIUM 

„ 

B  s 

200  -is  

"*  ••                  "Ni. 

*  x  k              ^ 

>  s  *       ^  s         \      ^  s 

x^ 

o       :::;:!±;_  !5;:: 

5  s^ 

N***ka        >Jn  4    - 

a           ^    <s 

•"  100               ^         's< 

^VN                                                                                ""    "~H-J"*L. 

^X*.                                               1^  *  -4* 

H!                                   s                    ^« 

*_-          _       _^5a^_      ..      J^Q  g   _  _|_ 

•*•  ^^^ 

s      "Mo  1 

'"SK«- 

""•i. 

/\                                   ^v 

I 

I 

4         5          G         1 
Stroke  in  Iiiches 


10       11       12 


FIG.  53. — Pull  of  cushion  type  of  electromagnet. 

53.  Lifting  and  holding  magnets  are  generally  of  the  horse- 
shoe or  of  the  annular  type  shown  diagrammatically  in  Figs.  54 
and  55.  As  the  iron  to  be  lifted  moves  from  a  to  &,  Fig.  55,  there 


M 


FIG.  54. — Horseshoe  type  of 
electromagnet. 


FIG.  55. — Annular  type  of 
electromagnet. 


is  little  change  in  the  flux  threading  the  coil  M  and  therefore  only 
a  small  pull;  when  the  iron  approaches  close  to  the  poles  of  the 
magnet,  however,  the  flux  increases  rapidly  and  the  pull,  being 
proportional  to  the  space  rate  of  change  of  flux,  becomes  large. 


44       PRINCIPLES  OF  ELECTRICAL  ENGINEERING      [CHAP,  vm 


For  such  magnets  the  holding  power  may  be  determined  very 
closely  by  Maxwell's  formula 


pull  in  dynes  =  - 

"  8?r 

where  (B  is  the  flux  density  across  the  contact  surface  in  lines  per 

sq.  cm. 
A  is  the  total  pole  face  area  in  sq.  cm. 

In  the  case  of  the  magnet  shown  in  Fig.  56,  the  scale  on  the  iron  to  be  lifted 
is  assumed  to  be  0.05  cm.  thick,  it  is  required  to  determine  how  the  pull  varies 
with  the  exciting  current. 

To  solve  this  problem  it  is  necessary  to  assume  different  values  for  the 
total  flux  in  the  magnetic  circuit  then  calculate  the  pull  by  the  use  of  the 
above  formula  and  the  excitation  by  the  use  of  the  curves  in  Fig.  41,  page  34. 


ISO 


A' 


it  Steel 


UXi 


t -J 


1.0  2.0  3.0 

Amperes 

FIG.  56. — Pull  of  a  horseshoe  magnet. 

lit  the  length  of  the  cast  steel  path  =  20  cm. 

lz,  the  length  of  each  air  gap  =  0.05  cm. 

1 3,  the  length  of  the  cast  iron  path  =  12  cm. 

Ai,  the  cross  section  of  the  cast  steel  path  =  2X4  =  8  sq.  cm. 

A 2,  the  cross  section  of  each  air  path  =  8  sq.  cm. 

A3,  the  cross  section  of  the  cast  iron  path  =  4X5  =  20  sq.  cm. 

If  0,  the  total  flux  =  80,000  lines 

then    (Bi,  the  flux  density  in  the  cast  steel  =  10,000  lines  per  sq.  cm. 
(B2,  the  flux  density  in  the  air  gaps     =  10,000  lines  per  sq.  cm. 
(B3,  the  flux  density  in  the  cast  iron   =    4,000  lines  per  sq.  cm. 
and  the  ampere  turns  per  cm.  for  the  cast  steel  =  7,  see  Fig.  41. 

the  ampere  turns  per  cm.  for  the  air  gaps  = -r ,  see  page  33. 

=  8000 


ART.  53] 


SOLENOIDS  AND  ELECTROMAGNETS 


45 


the  ampere  turns  per  cm.  for  the  cast  iron  =  13,  see  Fig.  41 

and  the  total  ampere  turns  =  7  X  20  +  8000  X  2  X  0.05  +  13  X  12 

=  140  +  800  +  156 

=  1096 


FIG.  57. — Annular*  type  of  electromagnet, 
the  magnetic  pull 


(10,000)2  X  2  X  8  , 

-~    -  dynes 


=  64,000,000  dynes 
=  65,000  gm. 
=  65  kg. 


46       PRINCIPLES  OF  ELECTRICAL  ENGINEERING       [CHAP,  vm 


Other  values  are  worked  out  in  the  same  way,  the  work  generally  being 
carried  out  in  tabular  form  as  below: 


Flux  density 

Ampere  turns  per        _,   , 
centimeter           .  Total  ampere  turns 

Am- 

<t> 

Steel       Air 

Iron 

| 

Steel  !    Air 

Iron    Steel     Air       Iron     C"" 

peres 

i 

! 

CUlt 

64,000 

8,000     8,000 

3,200 

5       6,400 

9 

100      640    108        848 

0.848 

42kg. 

80,000 

10,000  10,000 

4,000 

7       8,000 

13 

140      800    156     1,096 

1.096 

65kg. 

96,000 

12,000   12,000 

4,800 

11       9,600 

19 

222      960    228     1,408 

1.408 

94  kg. 

112,000 

14,000  14,000 

5,600 

19     11,200 

25 

380  1,120    300     1,800 

1.800 

126  kg. 

128,000 

16,000  16,000 

6,400 

55  i  12,800 

34      1,100  1,280    408     2,788 

2.788 

164  kg. 

These  results  are  plotted  in  Fig.  56. 

The  possibilities  of  the  annular  type  of  magnet  are  illustrated 
in  Fig.  57.  A  magnet  which  weighs  2250  Ib.  will  lift  skull  cracker 
balls  up  to  12,000  Ib.,  billets  and  slabs  up  to  20,000  Ib.  and  mis- 
cellaneous scrap  up  to  500  Ib.  The  power  required  to  operate  the 
magnet  being  1 1  amp.  at  220  volts  or  "2 . 42  kw. 

54.  Saturation  of  a  Magnetic  Circuit. — From  the  figures  in  the 
last  problem,  under  the  heading  of  total  ampere  turns,  it  may  be 
noted  that,  when  the  flux  densities  in  the  steel  and  iron  are  low, 
most  of  the  excitation  is  required  for  the  air  path  or  most  of  the 
reluctance  of  the  magnetic  circuit  is  in  the  air  gap. 

When  the  densities  exceed  15,000  lines  per  sq.  cm.  for  cast  steel 
and  6000  for  cast  iron,  the  reluctance  of  the  magnetic  circuit 
increases  rapidly  and  the  curve  showing  the  relation  between  flux 
and  excitation,  called  the  magnetization  curve  of  the  circuit, 
bends  over  rapidly  as  shown  in  Fig.  56,  the  circuit  is  then  said  to 
be  nearly  saturated. 

55.  Electromagnetic  Brakes  and  Clutches. — One  type  of  brake 
used  on  crane  motors  is  shown  in  Fig.  58.     The  annular  steel 
frame  A  of  the  electromagnet  is  fastened  to  the  housing  of  the 
motor  and  carries  the  exciting  coil  E.     The  sliding  disc  B  is 
fastened  to  the  frame  A  of  the  magnet  by  means  of  a  sliding  key 
F  and  is  free  to  move  axially  but  cannot  rotate. 

When  the  motor  is  disconnected,  the  magnet  is  not  excited  and 
the  springs  S  push  the  disc  B  into  the  ring  C  which  is  keyed  to  the 
motor  shaft,  the  motor  is  thereby  braked  and  brought  rapidly  to 
rest.  When  current  is  applied  to  start  the  motor,  the  coil  E  is 
excited  at  the  same  time  and  the  disc  B  is  attracted,  releasing  the 
ring  C,  so  that  the  motor  shaft  is  then  free  to  rotate.  Electro- 
magnetic clutches  are  built  on  the  same  principle. 


ART.  56] 


SOLENOIDS  AND  ELECTROMAGNETS 


47 


56.  Magnetic  Separator. — A  useful  application  of  the  electro- 
magnet is  shown  diagrammatically  in  Fig.  59.     The  magnetic 


FIG.  58. — Electromagnetic  brake. 

pulley  consists  of  an  iron  shell  containing  an  exciting  coil  C  which 
produces  the  magnetic  field  shown.  Any  iron  particles  carried 
over  this  pulley  by  the  conveyer  belt  are  attracted  and  are 


FIG.  59. — Magnetic  separator. 

therefore  carried  further  round  than  the  nonmagnetic  materials 
with  which  they  are  mixed. 


CHAPTER  IX 

ARMATURE  WINDINGS  FOR  DIRECT -CURRENT 
MACHINERY 

57.  Principle  of  Operation  of  the  Electric  Generator. — The 

simplest  type  of  electric  generator  is  shown  diagrammatically  in 
Fig.  60.  If  the  conductor  ab  is  moved  alternately  up  and  down  so 
as  to  cut  the  lines  of  force  that  pass  from  N  to  S,  an  e.m.f .  will 
be  generated  or  induced  in  the  conductor  which  will  cause  an 
electric  current  to  flow  in  the  closed  circuit  abed. 

The  direction  of  the  current  in  the  conductor  ab  may  be  deter- 
mined by  the  right-hand  rule,  page  9.     The  current  will  reverse 


FIG.  60. — Generation  of  electromotive  force. 

when  the  direction  of  motion  of  the  conductor  is  reversed,  so  that 
the  current  will  flow  first  in  one  direction  and  then  in  the  other; 
such  a  current  is  said  to  be  alternating. 

58.  Gramme  Ring  Winding. — The  first  satisfactory  machine 
that  would  give  a  direct  current,  that  is  a  current  which  flows  con- 
tinuously in  one  direction,  is  shown  diagrammatically  in  Fig.  61. 
Since  this  diagram  is  rather  complicated,  the  stages  in  its  develop- 
ment will  be  taken  up. 

The  poles  NS,  Fig.  62,  are  bored  out  cylindrically  and  then,  in 
order  to  reduce  the  reluctance  of  the  magnetic  circuit,  a  soft  iron 
core  is  placed  concentrically  with  the  pole  faces  so  as  to  make  the 
air  gap  clearances  a  and  b  small.  In  these  air  gaps  the  conductors 

48 


ART.  58] 


ARMATURE  WINDINGS 


49 


c  are  moved  in  the  direction  of  the  arrow  so  as  to  pass  down  in 
front  of  the  N  pole  and  up  in  front  of  the  S  pole  and  thereby  cut 
the  lines  of  force  that  pass  from  N  to  S,  so  that  e.m.fs.  are  gener- 
ated in  these  conductors,  the  direction  of  which,  determined  by 
the  right  hand  rule,  is  shown  in  Fig.  62  at  a  particular  instant. 

The  next  step  in  the  development  of  the  machine  is  to  attach 
the  conductors  c  to  the  central  core  as  shown  in  Fig.  63,  so  that 


FIG.  61. — Gramme  ring  winding. 

they  are  carried  around  when  the  core  is  driven  by  a  prime  mover; 
the  lines  of  force  still  pass  from  N  io  S  and  do  not  rotate  with  the 
core.  When  this  construction  is  used  it  is  found  necessary  to 
laminate  the  core  for  the  reason  given  in  art.  62,  page  55.  The 
core  area  should  be  large  enough  to  keep  the.  flux  density  below 
the  saturation  point,  see  page  46,  so  that,  while  the  centre  of  the 


o 

FIG.  62.  FIG.  63. 

Stages  in  the  development  of  the  Gramme  ring  winding. 

core  may  generally  be  cut  out  as  shown  so  as  to  save  material, 
the  depth  d  must  not  be  made  too  small. 

It  is  now  necessary  to  connect  the  individual  conductors  to- 
gether so  that  their  voltages  add  up  and  this  is  done  by  joining 
them  as  shown  in  Fig.  64  so  as  to  form  an  endless  helix.  The 
complete  core  and  winding  form  what  is  called  the  armature 
of  the  machine. 

4 


50          PRINCIPLES  OF  ELECTRICAL  ENGINEERING      [CHAP,  ix 

Since  the  lines  of  force  pass  through  the  iron  core  as  shown 
rather  than  directly  across  the  central  air  space  from  m  to  n, 
only  the  face  conductors  c  cut  these  lines,  no  lines  being  cut  by  the 
inner  conductors  e.  The  direction  of  the  e.m.f.  in  each  conductor 
is  indicated  by  crosses  and  dots  in  the  usual  way  and  it  may  be  seen 
that  no  current  can  flow  through  this  closed  winding  because  the 
voltages  in  the  conductors  under  the  N  pole  are  opposed  by  those 
in  the  conductors  under  the  S  pole.  A  difference  of  potential 
however  will  be  found  between  /  and  g  so  that  if  stationary  con- 
tacts placed  at  these  two  points  are  connected  to  an  external  cir- 
cuit as  in  Fig.  65,  then  current  will  flow  through  this  circuit  and 
back  through  the  two  paths  of  the  winding  as  shown.  As  long 
as  the  generator  rotates  in  the  direction  of  the  arrow,  the  voltage 


FIG.  64.  FIG.  65. 

Stages  in  the  development  of  the  Gramme  ring  winding. 

distribution  will  always  be  as  indicated  and  the  voltage  between 
the  points  /  and  g  will  be  constant  in  magnitude  and  direction. 

59.  Commutator  and  Brushes. — Machines  have  been  con- 
structed in  which  the  stationary  contacts  #_  and  B+,  called  the 
brushes,  were  allowed  to  rub  directly  on  the  winding  as  shown  in 
Fig.  65,  but  the  standard  practice  is  to  provide  a  special  rubbing 
contact  on  each  coil  such  as  that  shown  dotted  at  s,  Fig.  65.  The 
complete  winding  supplied  with  these  contacts  is  shown  diagram- 
matically  in  Fig.  61  and  is  also  shown  in  Fig.  66.  These  rubbing 
contacts  form  what  is  called  the  commutator,  and  the  individual 
contacts  are  called  the  commutator  segments. 

In  Fig.  61,  current  enters  the  machine  at  the  negative  brush 
#_,  passes  through  the  commutator  leads  a  to  the  winding 


ART.  60] 


ARMATURE  WINDINGS 


51 


through  which  it  passes  to  the  positive  brush  B+  and  then  on  to 
the  external  circuit.  The  voltage  between  #_  and  B+  is  main- 
tained so  long  as  the  armature  conductors  cut  lines  of  force,  while 


FIG.  66. — Armature  with  a  Gramme  ring  winding. 

the  amount  of  current  passing  through  the  machine  depends 
entirely  on  the  resistance  R  of  the  external  circuit. 

60.  Multipolar  Windings. — It  has  been  found  economical  in 


Incomplete  Winding 

FIG.  67. 


Complete  Winding 

FIG.  68. 


Diagrammatic  Representation  of  a  4  Pole  Winding 

FIG.  69. 
Gramme  ring  winding  for  a  four  pole  machine. 

practice  to  build  machines  with  more  than  two  poles,  see  page 
56,  the  poles  being  arranged  in  pairs  alternately  N  and  S.  In 
Fig.  68  the  winding  for  a  four-pole  machine  is  shown  diagrammat- 


52          PRINCIPLES  OF  ELECTRICAL  ENGINEERING       [CHAP,  ix 

ically.  The  direction  of  the  lines  of  force  and  of  the  e.m.f.  in  the 
conductors  is  shown  in  Fig.  67,  from  which  diagram  it  may  be 
seen  that  no  current  can  flow  in  the  closed  winding  because  the 
voltages  in  the  conductors  under  the.N  poles  are  opposed  by 
equal  voltages  in  the  conductors  under  the  S  poles.  A  difference 
of  potential  however  will  be  found  between  a  and  b  due  to  the 
conductors  cutting  lines  of  force  under  pole  Si  and  there  is  an 
equal  difference  of  potential  between  a  and  d  due  to  the  conduct- 
ors cutting  lines  of  force  under  pole  Nz  so  that  b  and  d  are  at  the 
same  potential  and  may  be  connected  together.  For  the  same 
reason  the  stationary  contacts  a  and  c  may  also  be  connected 
together  as  shown  in  Fig.  68.  The  external  circuit  to  be  supplied 
with  current  is  connected  between  the  terminals  T+  and  T_. 
This  current  will  divide  when  it  enters  the  machine  and  pass 
through  the  four  paths  in  the  winding  as  shown  in  Fig.  68  and 
also  diagrammat ically  in  Fig.  69. 

61.  Drum  Windings. — One  obvious  objection  to  the  ring  wind- 
ing is  that  only  the  outer  conductors  c,  Fig.  63,  cut  lines  of  force, 


FIG.  70. — Coil  of  a  drum  winding. 

the  remainder  of  the  winding  being  inactive.  To  overcome  this 
objection  most  modern  machines  have  what  is  called  a  drum  wind- 
ing made  with  coils  which  are  shaped  as  shown  in  Fig.  70  and  are 
placed  on  the  surface  of  the  armature  core  in  such  a  way  that  the 
conductors  a  and  b  are  under  poles  of  opposite  polarity.  The 
e.m.fs.  generated  in  these  conductors  therefore  act  in  the  same 
direction  around  the  coil. 

In  Fig.  71,  two  four-pole  machines  are  shown  which  are  alike 
in  every  respect  except  that  machine  A  has  a  ring  coil  between 
two  adjacent  commutator  segments  whereas  machine  B  has  a 
drum  coil. 

In  Fig.  72,  the  same  two  machines  are  shown  with  quarter  of 
the  armature  winding  in  place,  the  conductors  being  numbered  in 
the  order  in  which  they  are  connected  in  series.  By  following 


ART.  61] 


ARMATURE  WINDINGS 


53 


each  winding  from  the  negative  to  the  positive  brush,  it  will  be 
found  that  in  each  case  the  e.m.f.  between  the  brushes  is  due 


A.  Gramme  ring  coil.  B.  Drum  coil. 

FIG.  71. — Comparison  between  ring  and  drum  coils. 


A.  Gramme  ring  winding.  B.  Drum  winding. 

FIG.  72. — Comparison  between  ring  and  drum  windings. 


FIG.  73. — Complete  winding.     FIG.  74. — Diagrammatic 

representation. 
Drum  winding  for  a  four  pole  machine. 

to  the  voltage  generated  in  four  conductors  in  series,  no  voltage 
being  generated  in  conductors  1  and  6  since  they  are  not  under 
the  poles  and  so  are  not  cutting  lines  of  force. 


54          PRINCIPLES  OF  ELECTRICAL  ENGINEERING      [CHAP,  ix 

Fig.  73  shows  the  complete  drum  winding,  which  has  four  paths 
between  the  negative  and  the  positive  terminals  of  the  machine 
and  may  therefore  be  represented  diagrammatically  by  Fig.  74. 


FIG.  75. — Armature  with  a  drum  winding. 

A  complete  drum  winding  has  the  appearance  shown  in  Fig.  75 
but,  so  far  as  the  generation  of  voltage  is  concerned,  it  produces 
the  same  result  as  the  ring  winding  shown  in  Fig.  66.  By  choos- 


FIG.  76. — Two  turn  coil  for  a  drum  winding. 

ing  a  suitable  strength  of  magnetic  field,  and  a  suitable  number  of 
conductors  in  series  between  negative  and  positive  brushes,  the 
designer  is  able  to  wind  armatures  for  different  voltages.  The 


ART.  62J 


ARMATURE  WINDINGS 


55 


coils  shown  in  the  diagrams  in  this  chapter  have  only  one  turn 
between  adjacent  commutator  segments  but,  in  order  that  the 
voltages  used  in  practice  may  be  attained,  it  is  generally  neces- 
sary to  make  the  coils  as  shown  in  Fig.  76  with  several  turns 
between  segments. 

62.  Lamination  of  the  Armature  Core. — Fig.  77  shows  an 
armature  core  on  which  may  be  placed  either  a  ring  or  a  drum 
winding.  If  this  core  is  made  of  a  solid  block  of  iron,  then,  as  it 


FIG.  77. — Eddy  currents  in  a  solid  armature  core.     FIG.  78. — Laminated 

armature  core. 

rotates,  e.m.fs.  are  induced  in  the  surface  layers  and  force  current 
through  the  iron  in  the  direction  shown,  which  direction  may 
be  determined  by  the  right-hand  rule.  These  currents  cannot  be 
collected  and  utilized  but  power  is  required  to  maintain  them. 

To  keep  these  eddy  currents  small,  a  high  resistance  is  placed 
in  their  path  by  laminating  the  core  as  in  Fig.  78,  the  laminations 
being  separated  from  one  another  by  varnish. 


CHAPTER  X 


CONSTRUCTION  AND  EXCITATION  OF  DIRECT- 
CURRENT  MACHINES 

63.  Multipolar  Construction. — Fig.  79  shows  a  two-pole  ma- 
chine and 'also  a  six-pole  machine  built  for  the  same  output,  the 
machines  having  the  same  armature  diameter  and  the  same  total 
number  of  lines  of  force  crossing  the  air  gaps.  The  armature  core 
of  the  two-pole  machine  must  be  deep  enough  to  carry  half  of  the 
total  flux,  while  in  the  six-pole  machine  the  total  flux  divides  up 
among  six  paths  so  that  the  core  need  be  only  one-third  of  the 


Two-pole  machine.  Six-pole  machine. 

FIG.  79. — Machines  with  the  same  output. 

depth  of  that  of  the  two-pole  machine.     For  the  same  reason  the 
six-pole  machine  has  the  smaller  cross  section  of  yoke. 

By  the  use  of  the  multipolar  construction  therefore  there  is  a 
considerable  saving  in  material,  but  this  is  at  the  expense  of  an 
increase  in  the  cost  of  labor  because  of  the  increased  number  of 
parts  to  be  machined  and  handled.  The  number  of  poles  is 
chosen  by  the  designer  to  give  the  cheapest  machine  that  will 
operate  satisfactorily. 

56 


ART.  64]  CONSTRUCTION  AND  EXCITATION 


57 


58  PRINCIPLES  OF  ELECTRICAL  ENGINEERING    [CHAP,  x 

64.  Armature  Construction.— Fig.  80  shows  the  type  of  con- 
struction generally  adopted  for  small  machines.     The  armature 
core  M  is  built  up  of  sheet  steel  laminations  which  are  separated 
from  one  another  by  layers  of  varnish;  see  page  55. 

The  winding  shown  is  of  the  drum  type,  see  Fig.  75,  page  54, 
and  the  armature  coils  G  are  carried  in  slots  F  from  which  they 
are  insulated  by  paper,  cotton  and  mica.  It  is  found  that,  even 
when  embedded  in  slots,  the  conductors  cut  the  lines  of  force 
crossing  from  pole  to  pole. 

The  core  is  divided  into  sections  by  spacers  P,  so  that  air  can 
circulate  freely  through  the  machine  and  keep  it  cool.  The  core 
laminations  and  the  spacers  P  are^clamped  between  end  heads  N 
which  carry  coil  supports  L  attached  by  arms  shaped  like  fans. 
The  coils  are  held  against  these  supports  by  steel  band  wires  W. 

65.  Commutator. — The  commutator  is  built  of  segments  J, 
see   page  50,  which  are  of   hard-drawn    copper.     These    seg- 
ments are  separated  from  one  another  by  mica  strips  and  are  then 
clamped  between  two  cones  S  from  which  they  are  separated  by 
mica,  the  segments  being  thereby  insulated  from  one  another 
and  from  the  frame  of  the  machine.     The  segments  are  connected 
to  the  winding  through  the  leads  H  which,  in  modern  machines, 
have  air  spaces  between  one  another  as  shown,  so  that  air  is 
drawn  across  the  commutator  and  between  the  leads  thereby 
keeping  the  commutator  cool. 

66.  The  brushes,  see  page  50,  are  attached  to  the  studs  X, 
which  studs  are  insulated  from  the  supporting  arm  V,  and  con- 
nection is  made  from  these  studs  to  the  external  circuit. 

67.  Poles  and  Yoke. — The  armature  revolves  in  the  magnetic 
field  produced  by  the  exciting  or  field  coils  A  which  are  wound  on 
the  poles  B.     These  poles  must  have  sufficient  cross  section  to 
carry  the  magnetic  flux  without  the  flux  density  becoming  too 
high,  the  same  applies  to  the  cast  steel  yoke  C  to  which  the  poles 
are  attached  by  screws. 

68.  Large  generators  are  similar  to  small  generators  such  as 
that  described  above;  some  changes  are  generally  required  in  the 
mechanical  design  because  of  the  heavier  parts  to  be  supported 
and  also  because  of  the  different  kinds  of  service  for  which  ma- 
chines have  to  be  built.     In  the  case  of  the  engine  type  generator 
shown  in  Fig.  81  for  example,  the  armature  core  is  built  up  of 
segments  instead  of  complete  rings,  while  the  commutator  is 


ART.  69] 


CONSTRUCTION  AND  EXCITATION 


59 


supported  from  the  armature  spider  since  the  shaft  is  supplied  by 
the  engine  builder. 


Stud 


FIG.  81. — Large  direct-current  generator. 


FIG.  82. — Separately  excited  FIG.  83. — Shunt  excited 

machine.  machine. 


69.  Excitation. — Permanent  magnets  are  used  as  field  poles 
for  small  machines  called  magnetos;  large  machines  are  supplied 


60 


PRINCIPLES  OF  ELECTRICAL  ENGINEERING       [CHAP,  x 


with  electromagnets  the  excitation  of  which  can  readily  be 
controlled. 

When  the  generator  itself  supplies  this  exciting  current  it  is  said 
to  be  self  excited;  when  the  exciting  current  is  supplied  from 
some  external  source  the  machine  is  said  to  be  separately  excited. 
The  different  connections  used  are  shown  in  Figs.  82,  83,  84 
and  85. 

Fig.  82  shows  a  separately  excited  machine. 

Fig.  83  shows  a  shunt  machine  in  which  the  field  coils  form  a 


Short  Shunt 


FIG.  84. — Series  excited 
machine. 


Long  Shunt 


FIG.  85. — Compound  excited 
machine. 


shunt  across  the  armature  terminals  and  have  many  turns  of  small 
wire  carrying  a  current  //  =  Et/Rf,  the  terminal  voltage  divided 
by  the  resistance  of  the  field  coil  circuit.  This  exciting  current 
seldom  exceeds  5  per  cent,  of  the  full-load  current  supplied  to  the 
external  circuit. 

Fig.  84  shows  a  series  machine  in  which  the  field  coils  are  in 
series  with  the  armature  and  have  only  about  5  per  cent,  of  the 
number  of  turns  that  a  shunt  winding  would  have,  but  employ  a 
larger  size  of  wire  because  they  have  to  carry  the  total  current  of 
the  machine. 


ART.  69]  CONSTRUCTION  AND  EXCITATION  61 

Fig.  85  shows  a  compound  machine  in  which  there  are  both 
shunt  and  series  field  coils.  When  the  shunt  coils  are  connected 
outside  of  the  series  coils  the  machine  is  said  to  have  a  long  shunt 
connection;  when  connected  inside  of  the  series  coils  the  connec- 
tion is  said  to  be  short  shunt. 


CHAPTER  XI 
THEORY  OF  COMMUTATION 

70.  Commutation. — As  the  armature  of  a  direct-current  genera- 
tor revolves,  the  direction  of  the  current  in  each  conductor 
changes  while  that  conductor  passes  from  one  pole  to  that 
adjoining.  In  Fig.  86  for  example,  the  direction  of  the  current 
in  the  coil  M  is  shown  at  three  consecutive  instants  in  diagrams 
A,  B  and  C. 

As  the  armature  moves  from  A  to  C  the  brush  changes  from  seg- 


FIG.  86. — Diagram  showing  the  reversal  of  the  current  in  coil  M. 

ment  1  to  segment  2  and  the  current  in  the  coil  M  is  automatically 
reversed.  For  a  short  period,  as  at  B,  the  brush  is  in  contact  with 
both  segments  and,  during  this  interval  of  time,  the  coil  M  is  short 
circuited,  but  no  e.m.f.  is  generated  in  the  coil  since  it  is  not 
cutting  lines  of  force,  so  that  no  current  passes  through  the  short 
circuit. 

When  in  position  B,  the  coil  is  said  to  be  in  the  neutral  position 
and  the  line  L  is  called  the  neutral  line. 

The  operation  of  reversing  the  current  in  an  armature  coil  by 

62 


ART.  71]  THEORY  OF  COMMUTATION  63 

means  of  the  brush  and  commutator  segments  is  called  commuta- 
tion. Unfortunately  the  operation  is  not  so  simple  as  described 
above,  because  the  coils  have  self  induction  and  resist  a  change  of 
current,  and  this  we  shall  see  causes  sparking  and  gradual  deteri- 
oration of  the  brushes  and  commutator.  It  is  therefore  neces- 
sary to  make  a  detailed  study  of  the  subject  because  of  its 
importance. 

To  study  the  variation  of  the  current  in  the  coil  being  corn- 
mutated,  the  student  should  draw  the  brushes  B+  and  B_  and 
also  the  poles  N  and  $,  Fig.  87,  on  a  piece  of  heavy  paper,  and  the 
armature  and  commutator  on  tracing  paper.  The  armature 
should  then  be  placed  in  the  magnetic  field  and  the  direction 
of  the  current  in  a  particular  coil  noted  as  the  armature 
goes  through  one  revolution.  Such  a  model  illustrates  the 
operation  much  better  than  any  set  of  diagrams  such  as  those 
in  Figs.  88,  89  and  90. 

71.  Theory  of  Commutation. — Fig.  88  shows  part  of  a  machine 
with  a  ring  winding  having  two  turns  per  coil  and  with  the  current 
in  the  coil  M  undergoing  commutation.     The  brush  B  is  made  of 
copper  so  that  the  resistance  of  the  contact  between  the  brush  and 
the  commutator  is  negligible. 

In  diagram  A,  the  currents  I  enter  the  brush  through  the  com- 
mutator lead  a. 

In  diagram  B,  the  brush  makes  contact  with  two  segments  and 
the  current  flowing  to  the  brush  through  the  coils  under  the  S 
pole  no  longer  requires  to  flow  round  coil  M  because  it  has  an 
easier  path  through  the  lead  b,  the  current  in  coil  M  therefore  dies 
down  to  zero  because,  being  in  the  neutral  position,  the  coil  M  is 
not  cutting  lines  of  force  so  that  no  e.m.f.  is  generated  in  it  to 
maintain  the  current. 

In  diagram  D,  segment  1  of  the  commutator  is  about  to  break 
contact  with  the  brush,  and  the  coil  M  carrying  no  current  is 
about  to  be  thrown  in  series  with  the  coils  under  the  N  pole. 
At  the  instant  the  contact  is  broken,  as  shown  in  diagram  E,  the 
current  in  coil  M  tries  to  increase  suddenly  from  zero  to  a  value 
7,  but  this  change  of  current  is  opposed  by  the  self  induction  of 
the  coil  M,  so  that  the  current  prefers  to  pass  to  the  brush  across 
the  air  space  xt  causing  sparking. 

72.  Shifting  of  the  Brushes. — For  sparkless  commutation,  it 
is  necessary  that  the  current  in  the  coil  M  shall  be  reduced  to  zero 
and  then,  by  some  means  or  other,  raised  to  a  value  I  in  the  oppo- 


64          PRINCIPLES  OF  ELECTRICAL  ENGINEERING      [CHAP.XI 


FIG.  88.  FIG.  89.  FIG.  90. 

FIG.  88. — With  low  resistance  brushes  on  neutral  line. 
FIG.  89. — With  low  resistance  brushes  shifted  in  the  direction  of  motion. 
FIG. '90. — With  high  resistance  brushes. 

Stages  in  the  process  of  commutation. 


ART.  73] 


THEORY  OF  COMMUTATION 


65 


site  direction  during  the  time  the  coil  is  short  circuited  by  the 
brush,  so  that  when  the  contact  at  x  is  broken,  there  shall  be  no 
sudden  change  of  current  in  the  coil. 

This  result  may  be  obtained  by  shifting  the  brushes  forward 
in  the  direction  of  motion  as  shown  in  Fig.  89  so  that,  while  coil 
M  is  short  circuited,  it  is  in  a  magnetic  field  and  an  e.m.f.  is  gen- 
erated in  it  which  will  produce  the  required  growth  of  current. 
This  magnetic  field  is  called  the  reversing  field.  The  correspond- 
ing diagrams  in  Figs.  88  and  89,  and  particularly  diagrams  D 
and  E,  should  be  carefully  compared. 

If  the  current  taken  from  the  generator  is  increased,  the 
strength  of  the  reversing  field  must  also  be  increased  if  commuta- 
tion is  to  be  sparkless,  so  that  the  brushes  must  be  moved  nearer 
to  the  pole  tips  and  further  from  the  no-load  position.  The 
brush  position  must  therefore  be  changed  with  change  of  load. 

73.  Interpole  Machines. — It  has  been  shown  in  the  last  para- 
graph that  the  commutation  of  a  generator  is  improved  if  the 


FIG.  91.  FIG.  92. 

Diagrams  illustrating  the  principle  of  the  interpole  generator. 

brushes  are  shifted  forward  in  the  direction  of  motion  of  the 
machine  so  that  the  short  circuited  coils  are  in  a  reversing  mag- 
netic field,  thus  in  Fig.  91  the  brush  B+  is  moved  so  as  to  come 
under  the  tip  of  the  N  pole  and  the  brush  B_  is  moved  so  as  to 
come  under  the  tip  of  the  S  pole.  The  same  result  may  be  accom- 
plished by  leaving  the  brushes  in  the  neutral  position  and 
bringing  an  auxiliary  n  pole  over  the  brush  B+  and  an  auxiliary  s 
pole  over  the  brush  B_,  as  shown  in  Fig.  92.  These  auxiliary 
poles  are  called  interpoles  and  a  machine  so  equipped  is  called  an 
interpole  machine. 

It  is  desirable  that  the  strength  of  the  reversing  field  increase 
with  the  current  drawn  from  the  armature  and  to  obtain  this 

5 


66        PRINCIPLES  OF  ELECTRICAL  ENGINEERING        [CHAP,  xi 

result  the  interpoles  are  supplied  with  series  field  coils  as  shown 
in  Fig.  92. 

74.  Carbon  brushes  have  a  contact  resistance  which  is  gener- 
ally about  ten  times  that  of  copper  brushes.  The  effect  of  this 
high  contact  resistance  is  to  improve  commutation,  as  may  be 
seen  by  a  comparison  between  Figs.  88  and  90. 

In  diagram  B,  Fig.  90,  the  brush  makes  contact  with  segment  2 
and  some  of  the  current  /  that  was  flowing  round  coil  M  now 
flows  directly  to  the  brush  through  lead  b.  Since  however  the 
contact  with  segment  2  is  small  in  area,  the  current  ib  flowing 
through  this  contact  is  small  and  some  of  the  current  I  continues 
to  flow  around  coil  M . 

As  the  armature  rotates  and  the  contact  area  between  the 
brush  and  segment  2  increases,  the  current  ib  increases  and  that 
in  coil  M  decreases  until,  at  the  instant  shown  in  diagram  C, 
the  current  in  this  coil  has  become  zero. 

In  diagram  D,  the  contact  area  between  the  brush  and  segment  1 
is  small  while  that  between  segment  2  and  the  brush  is  large  so 
that  the  current  in  lead  a  is  throttled  by  the  high  resistance  of  the 
small  contact  area  and  current  is  forced  around  coil  M  in  the 
direction  shown.  As  the  contact  area  between  the  brush  and 
segment  1  decreases,  the  current  ia  decreases  and  that  in  coil  M 
increases  until,  at  the  instant  shown  in  diagram  E,  when  the 
contact  at  x  is  broken,  the  current  in  coil  M  has  been  raised  to  the 
value  I  by  this  slide  valve  action  of  the  high  resistance  brush. 
The  contact  can  then  be  broken  without  causing  any  sudden 
change  of  current  in  the  coil  M  and  therefore  without  sparking. 

In  the  above  theory,  the  action  at  the  positive  brush  has  been 
considered;  the  action  at  the  negative  brush  is  similar  and  need 
not  be  considered  separately.  The  theory  applies  to  a  drum 
winding  as  well  as  to  a  ring  winding,  the  only  difference  between 
the  two  cases  being  in  the  shape  of  the  coil,  see  Fig.  71. 

By  the  use  of  carbon  brushes  it  is  possible  to  operate  generators 
from  no-load  to  full-load  without  shifting  of  the  brushes  during 
operation,  and  for  that  reason  carbon  brushes  have  superseded 
copper  brushes  on  modern  machines. 


CHAPTER  XII 
ARMATURE  REACTION 

75.  The  Cross-magnetizing  Effect — In  Fig.  93,  diagram  A 
shows  the  distribution  of  the  magnetic  flux  in  a  two-pole  machine 
when  the  field  coils  are  excited  and  no  current  is  flowing  in  the 
armature  winding;  the  flux  density  is  uniform  under  the  pole  face 
so  that  the  same  number  of  lines  of  force  cross  each  square  centi- 
meter of  the  air  gap  between  the  pole  face  and  the  armature 
surface. 

Diagram  B  shows  the  distribution  of  magnetic  flux  when  the 
armature  is  carrying  current,  the  brushes  being  in  the  neutral 


Flux  distribution  due 
to  the  field  coils 

FIG.  93. 


B  C 

Flux  distribution  due  to  the 
armature  winding 

Armature  reaction  with  the  brushes  in  the  neutral  position. 


Resultant  flux 
distribution 


position  and  the  field  coils  not  excited.  The  current  passing 
downward  in  the  conductors  under  the  S  pole  of  the  machine  and 
up  in  those  which  are  under  the  N  pole  causes  the  armature  to 
become  an  electromagnet  with  lines  of  force  which  pass  through 
the  armature  in  a  direction  determined  by  the  corkscrew 
law,  see  page  5,  and  which  return  across  the  pole  faces  to  com- 
plete the  circuit. 

Diagram  C  shows  the  resultant  distribution  of  magnetic 
flux  when,  as  under  load  conditions,  the  armature  is  carry- 
ing current  and  the  field  coils  are  excited;  C  is  obtained  by 
combining  the  magnetic  fields  of  A  and  B.  Under  pole  tips  a  and 
c  the  magnetic  field  due  to  the  current  in  the  armature  is  opposite 

67 


68 


PRINCIPLES  OF  ELECTRICAL  ENGINEERING     [CHAP,  xn 


in  direction  to  that  due  to  the  current  in  the  field  coils  while  under 
tips  b  and  d  the  two  magnetic  fields  are  in  the  same  direction. 

Since  the  armature  magnetic  field  is  at  right  angles  to  that  pro- 
duced by  the  field  magnets,  the  effect  produced  is  called  the  cross- 
magnetizing  effect  of  armature  reaction. 

76.  The  Demagnetizing  Effect. — In  a  direct -current  generator 
the  brushes  are  shifted  from  the  no-load  neutral  in  the  direction  of 
motion  so  as  to  improve  commutation,  see  page  63,  the  distribu- 
tion of  the  magnetic  flux  when  the  armature  is  carrying  current 
and  the  field  coils  are  not  excited  will  then  be  as  shown  in  diagram 
A,  Fig.  94.  The  armature  field  is  no  longer  at  right  angles  to  that 
produced  by  the  field  magnets  but  acts  in  the  direction  oz,  it 
may  however  be  considered  as  the  resultant  of  two  magnetic  fields, 
one  in  the  direction  oy,  called  the  cross-magnetizing  component 
and  the  other  in  the  direction  ox,  called  the  demagnetizing  com- 
ponent because  it  is  directly  opposed  to  the  field  produced  by  the 
field  magnets.  Diagram  B,  Fig.  94,  shows  the  armature  divided 


B 


FIG.  94. — Flux  distribution  due  to  the  armature  winding  when  the  brushes 
are  shifted  in  the  direction  of  motion. 

so  as  to  produce  these  two  components;  the  belts  of  conductors  ah 
and  cd,  when  carrying  current,  tend  to  demagnetize  the  machine, 
while  the  belts  ad  and  be  are  cross-magnetizing  in  effect. 

77.  Effect  of  Armature  Reaction  on  Commutation. — When 
interpoles  are  not  supplied,  the  brushes  are  shifted  forward  in  the 
direction  of  motion  so  that  commutation  takes  place  in  a  reversing 
magnetic  field  under  pole  tips  a  and  c,  Fig.  93.  But  it  may  be 
seen  from  diagram  C,  Fig.  93,  that  the  effect  of  armature  reaction 
is  to  weaken  the  magnetic  field  under  these  pole  tips  and  so  impair 
the  commutation. 

This  effect  must  be  minimized  by  making  the  air  gap  clear- 
ances 5  as  large  as  possible  so  that  there  is  a  large  reluctance  in  the 
path  of  the  cross  field.  Increasing  the  air  gap  also  increases 


ART.  77]  ARMATURE  REACTION  69 

the  reluctance  of  the  main  magnetic  path  and,  in  order  to  pro- 
duce the  required  main  flux,  it  is  then  necessary  to  increase 
the  number  of  exciting  ampere  turns  on  the  poles.  The  machine 
is  then  said  to  have  a  stiff  magnetic  field  because  it  is  not  greatly 
affected  by  armature  reaction. 


CHAPTER  XIII 
CHARACTERISTICS  OF  DIRECT -CURRENT  GENERATORS 

78.  Magnetization  or  No-load  Saturation  Curve. — The  voltage 
generated  in  the  armature  of  a  direct-current  machine,  being 
proportional  to  the  rate  of  cutting  lines  of  force,  is  proportional  to 
the  speed  and  to  the  flux  per  pole  or 

E  =  a  const.  X  <f>  X  r.p.m.  for  a  given  machine. 

The  flux  per  pole,  and  therefore  the  voltage,  increase  with  the 
excitation,  and  the  curve  showing  the  relation  between  no-load 
voltage  and  excitation,  the  speed  being  constant,  is  called  the  mag- 
netization or  the  no-load  saturation  curve  of  the  machine.  Such 


O  a  Exciting  Current    If 

FIG.  95. — Magnetization  curve  of  a  direct  current  generator. 

a  curve  is  shown  in  Fig.  95.  With  no  excitation  there  is  a  voltage 
er  due  to  residual  magnetism;  as  the  exciting  current  is  increased, 
the  flux  per  pole  and  the  voltage  increase  in  the  same  ratio  until, 
with  an  exciting  current  of  oa  amperes  the  magnetic  circuit  begins 
to  saturate  and  the  voltage  to  increase  more  slowly. 

To  obtain  such  a  curve  experimentally,  the  generator  is  driven 
at  a  constant  speed  with  no  connected  load.  The  field  coils 
are  separately  excited  as  shown  in  Fig.  95  and  the  exciting  cur- 
rent is  increased  by  gradually  cutting  out  the  resistance  r. 

70 


ART.  80] 


DIRECT-CURRENT  GENERATORS 


71 


Simultaneous  readings  of  the  voltage  E0  and,  the  current  //  are 
taken  and  the  results  plotted  as  shown. 

79.  Self  excitation  is  made  possible  by  virtue  of  the  residual 
magnetism  in  the  magnetic  circuit  of  the  machine.  If  for  example 
a  shunt  generator,  connected  as  in  Fig.  96,  is  rotating,  a  small  vol- 
tage er  is  generated  in  the  armature  even  with  no  exciting  current 
in  the  field  coils,  because  the  lines  of  force  of  residual  magnetism 
are  being  cut.  This  small  voltage  sends  a  small  current  through 
the  field  coils,  which  increases  the  magnetic  flux  and  causes  the 
generated  voltage  to  increase,  and  this  in  turn  further  increases 
the  excitation,  and  so  the  voltage  of  the  machine  builds  up. 

The  voltage  and  the  exciting  current  cannot  build  up  indefi- 
nitely because,  as  the  exciting  current  increases,  the  magnetic 


tf 


Exciting  Current 


FIG.  96 — Magnetization  curve  of  a  shunt  generator. 

circuit  becomes  more  nearly  saturated  and  the  voltage  increases 
by  a  smaller  and  smaller  amount  until  finally,  when  the  point  A  is 
reached  at  which  E0/If  =  R/,  the  voltage  and  exciting  current 
can  increase  no  further. 

It  frequently  happens  that,  when  a  generator  is  started  up  for 
the  first  time,  the  e.m.f.  generated  in  the  armature  due  to  residual 
magnetism  sends  a  current  through  the  field  coils  in  such  a. direc- 
tion as  to  oppose  the  residual  flux,  and  the  voltage,  instead  of 
building  up,  is  reduced  to  zero.  In  such  a  case  it  is  necessary 
to  reverse  the  connections  of  the  field  coils  so  as  to  pass  current 
through  them  in  the  opposite  direction. 

80.  Regulation  Curve  of  a  Separately  Excited  or  of  a  Magneto 
Generator. — This  curve,  sometimes  called  the  external  character- 


72        PRINCIPLES  OF  ELECTRICAL  ENGINEERING     [CHAP,  xm 


istic,  gives  the  relation  between  Et,  the  terminal  voltage,  -and  7a, 
the  line  current,  and  is  shown  in  Fig.  97  for  the  case  of  a  separately 
excited  machine  operating  at  constant  speed  and  with  constant 
excitation.  The  terminal  voltage  drops  as  the  current  taken  from 
the  machine  is  increased  because: 

a.  The  flux  per  pole  is  reduced  by  armature  reaction,  see  page 
68,  so  that  Eg,  the  e.m.f.  generated  by  cutting  this  flux,  is  also 
reduced. 

b.  The  terminal  voltage  Et  is  less  than  the  generated  voltage 
Eg  by  the  armature  resistance  drop  IaRa,  that  part  of  the  gener- 
ated voltage  required  to  force  the  armature  current  through  the 
resistance  of  the  armature  winding  and  of  the  brush  contacts. 


77^  Drop  Due  to  Armature  Reaction 

rmaturc  Resistance  Drop    Ia  Ra 


Line  Current 

FIG.  97. — Regulation  curve  of  a  separately  excited  generator. 

To  obtain  such  a  curve  experimentally,  the  generator  is  loaded 
on  a  bank  of  lamps,  or  some  other  suitable  load  that  can  readily  be 
adjusted,  as  shown  in  Fig.  97.  The  speed  and  the  exciting  cur- 
rent //  are  kept  constant,  while  the  current  taken  from  the 
machine  is  gradually  increased  by  connecting  an  increasing  num- 
ber of  lamps  in  parallel  across  the  terminals,  that  is  by  providing 
more  paths  through  which  current  can  pass.  Simultaneous 
readings  of  the  voltage  Et  and  of  the  current  Ia  are  taken  and  the 
results  plotted  as  in  Fig.  97. 

The  regulation  of  the  above  generator  is  defined  as  the  per  cent, 
change  in  voltage  when  full-load  is  thrown  off  the  machine,  the 
speed  and  the  field  circuit  being  unchanged.  The  regulation 
therefore  =  (E0  -  Et)/Et. 

81.  Regulation  Curve  of  a  Shunt  Generator. — This  curve  is 
shown  in  Fig.  98  for  a  constant  speed  shunt  excited  generator. 


ART.  81] 


DIRECT-CURRENT  GENERATORS 


73 


The  terminal  voltage  drops  as  the  current  taken  from  the  machine 
is  increased  because: 

a.  The  flux  per  pole  is  reduced  by  armature  reaction. 

6.  The  armature  drop  IaRa  is  used  up  in  the  machine  itself. 

c.  The  exciting  current  //  is  equal  to  Et/R/>  where  Rf  is  the 
constant  resistance  of  the  shunt  field  circuit,  so  that  as  the  ter- 
minal voltage  drops  the  exciting  current  decreases  and  causes  the 
voltage  to  drop  still  further.  Because  of  this  third  effect  the 
terminal  voltage  of  a  generator  with  a  given  load  will  be  lower 
when  the  machine  is  shunt  excited  than  when  separately  excited. 


1 .XDrop  Due  to  Armature 

"£r~--  —      Reaction 
1  ^Armature  Resistance 

Drop     Ia  Ra 
Drop  Due  to  Decrease 
in  Excitation 


FIG. 


Line  Current   /j  Ijn 

>. — Regulation  curve  of  a  shunt  generator. 


To  obtain  such  a  curve  experimentally  the  machine  is  connected 
up  as  shown.  The  speed  and  the  resistance  of  the  shunt  circuit 
are  kept  constant  while  the  current  taken  from  .the  machine  is 
gradually  increased,  and  simultaneous  readings  are  taken  of  the 
voltage  Et  and  the  current  //,  these  results  are  plotted  as  in 
Fig.  98. 

As  the  resistance  of  the  external  circuit  is  decreased,  the  current 
supplied  by  the  machine  increases  and  the  terminal  voltage 
drops  until  point  d  is  reached.  A  further  reduction  in  the  external 
resistance  allows  an  increased  current  to  flow  for  an  instant,  but 
this  increase  of  current  reacts  by  armature  reaction  and  causes 
such  a  large  drop  in  voltage  and  in  exciting  current  that  the  arma- 
ture current  cannot  be  maintained.  In  the  extreme  case  when 
the  generator  is  short  circuited,  that  is,  the  terminals  of  the  ma- 
chine are  connected  through  a  circuit  of  negligible  resistance,  then 
the  terminal  voltage  must  be  zero  and  there  can  be  no  field  excita- 


74       PRINCIPLES  OF  ELECTRICAL  ENGINEERING      [CHAP,  xm 


tion,  so  that  no  current  can  flow  in  the  short  circuit;  thus  c, 
Fig.  98,  is  a  point  on  the  load  curve. 

If  a  shunt  generator  is  short  circuited,  it  carries  the  maximum 
current  Im  for  only  a  short  interval- of  time  before  it  loses  its 
voltage  and  is  thereby  protected  from  injury.  For  the  same 
reason  a  shunt  generator  will  not  build  up  if  a  very  low  resistance 
is  connected  across  its  terminals.  Because  of  this  self  protecting 
power,  shunt  generators  are  used  with  advantage  for  electric  fur- 
nace and  other  work  during  the  process  of  which  the  machine  is 
liable  to  be  short  circuited. 

82.  To  maintain  the  terminal  voltage  constant,  shunt  gen- 
erators are  operated  with  an  adjustable  resistance,  called  a  field 


Over  Compound 
Generator 


Line  Current 


FIG.  99.  —  Regulation  curves  of  compound  generators. 

rheostat,  placed  in  the  field  coil  circuit.  As  the  load  on  the 
machine  increases  and  the  voltage  drops,  some  of  this  resistance 
may  be  cut  out  either  automatically  or  by  hand  so  as  to  increase 
the  excitation.  Automatic  regulators  for  this  purpose  are  used 
with  alternating-current  generators,  see  page  249,  but  are  sel- 
dom used  with  direct-current  generators  because  the  same  result 
may  be  obtained  more  cheaply  by  the  use  of  compound  windings. 
83.  Compound  generators,  operated  Without  a  regulator, 
maintain  the  terminal  voltage  approximately  constant  from  no- 
load  to  full-load,  because  the  line  current  passes  through  the  series 
field  coils  and  causes  the  total  excitation  to  increase  with  the 
load.  By  the  use  of  a  large  number  of  series  turns,  the  total 
excitation  may  increase  so  much  with  the  load  that  the  terminal 
voltage  will  rise,  as  shown  in  curve  B  Fig.  99,  the  machine  is  then 


ART.  84] 


DIRECT-CURRENT  GENERATORS 


75 


said  to  be  overcompounded.  When  the  terminal  voltage  has  the 
same  value  at  full-load  as  at  no-load,  the  machine  is  said  to  be  flat 
compounded. 

Generators  for  lighting  and  power  service  are  generally  flat- 
compound  machines  wound  for  125  or  for  250  volts.  For  railway 
service  the  generators  are  overcompounded  so  as  to  maintain  the 
trolley  voltage  at  some  distance  from  the  power  house.  Street 
railway  generators  are  invariably  wound  for  600  volts  at  full- 
load,  while  for  interurban  and  trunk  line  work  2400  volts  has  been 
used. 

84.  The  Regulation  Curve  of  a  Series  Generator. — Curve  A, 
Fig.  100,  shows  what  the  relation  between  voltage  and  current 


Line  Current 

FIG.  100. — Regulation  curve  of  a  series  generator. 

in  a  series  generator  would  be  if  armature  resistance  and  armature 
reaction  were  negligible;  the  voltage  would  increase  with  the  load 
current  since  this  is  also  the  exciting  current.  Curve  A  is  really 
the  no-load  saturation  curve  of  the  machine  and  is  determined  by 
separately  exciting  the  field  coils,  as  shown  in  diagram  A,  so  that 
no  current  flows  in  the  armature.  Curve  B  shows  the  actual  rela- 
tion between  terminal  voltage  and  load  current;  the  drop  of 
voltage  between  curves  A  and  B  consists  of  the  portion  due  to  the 
reduction  in  the  flux  per  pole  caused  by  armature  reaction,  and 
IaRa  the  drop  of  voltage  in  the  armature  winding,  brush  contacts 
and  series  field  coils. 

Series  generators  were  formerly  used  as  constant-current  gen- 
erators for  the  operation  of  arc  lamps  in  series.  They  were 
operated  with  automatic  regulators  so  as  to  have  the  line  ab,  Fig. 
100,  nearly  vertical,  and  the  current  practically  constant  for  all 
voltages  up  to  Em. 


76         PRINCIPLES  OF  ELECTRICAL  ENGINEERING     [CIIAP.XII. 


A  very  simple  type  of  regulator  for  this  purpose  is  shown  dia- 
grammatically  in  Fig.  101,  where  C  is  a  carbon  pile  rheostat  and 
B  is  a  solenoid  carrying  the  line  current.  If  the  current  in  the 
external  circuit  increases,  the  pull  of  the  solenoid  B  also  increases 
and  the  carbon  pile  is  compressed  and  its  resistance  thereby 
decreased,  see  page  29,  so  that  it  shunts  more  of  the  current  from 
the  series  field  coils.  The  flux  in  the  machine  is  therefore  reduced, 
and  the  voltage  drops  until  the  current  in  the  line  reaches  the 
value  for  which  the  pull  of  the  solenoid  was  adjusted. 


FIG.  101. — Automatic  regulator  for  a  constant- current  generator. 

85.  Problem  on  Generator  Characteristics. — a.  A  direct-current  shunt 
generator  was  tested  with  the  brushes  shifted  forward  in  the  direction  of 
motion.  The  voltage  drop  between  no-load  and  full-load  was  6  volts,  what 
are  the  causes  of  this  drop  in  voltage? 

6.  If  the  brushes  had  been  placed  on  the  neutral  position,  why  would  the 
voltage  drop  have  been  different  and  what  would  you  expect  its  value  to  be. 
Why  are  the  brushes  not  placed  on  the  neutral  position  in  non-interpole 
machines? 

c.  Series  field  coils  were  added  to  the  machine  and  the  voltage  dropped  20 
volts  from  no-load  to  full-load,  what  was  the  cause  of  this  excessive  drop  ? 

d.  After  the  series  field  coil  circuit  has  been  fixed,  the  voltage  was  found 
to  increase  by  6  volts  from  no-load  to  full-load  while  flat  compounding  was 
desired,  what  changes  would  you  suggest  should  now  be  made? 

a.  The  voltage  drop  is  due  to: 

1.  The  reduction  in  the  flux  per  pole  due  to  the  demagnetizing  effect  of 
armature  reaction,  since  the  brushes  are  shifted  forward. 

2.  The  armature  resistance  drop. 

3.  The  reduction  in  the  exciting  current  which  causes  the  flux  per  pole  to 
decrease  and  the  voltage  to  drop  still  further,  see  page  73. 

6.  When  the  brushes  are  placed  in  the  neutral  position,  the  armature  reac- 
tion has  no  demagnetizing  effect,  see  page  68,  so  that  the  voltage  drop  will  be 
less  than  when  the  brushes  are  shifted  forward,  and  will  probably  not  exceed 
4  volts. 


ART.  85] 


DIRECT-CURRENT  GENERATORS 


77 


cries  Shunt 


Machines  have  the  brushes  shifted  forward  in  order  to  improve  com- 
mutation and,  unless  interpoles  are  supplied,  these  machines  will  generally 
spark  at  the  commutator  on  full-load  if  the  brushes  are  kept  on  the  n'o-load 
neutral. 

c.  Since  the  voltage  drop  when  the  series 
field   was  added   was  greater  than  before, 
it  is  evident  that  this  field  has  been  con- 
nected    backward    so    as    to    oppose    the 
shunt  field  instead   of  assist  it,  the  series 
connections  must  therefore  be  reversed  so 
that  the  current  passes  through  the  series 
coils  in  the  proper  direction. 

d.  Since  the  compounding  effect  of  the 
series  coils  is  too  large,  it  will  be  necessary 
to  reduce  the  number  of  series  turns  or  to 
reduce  the   current  flowing    through  these 

turns.  The  latter  method  is  that  generally  adopted,  a  shunt  being  placed 
in  parallel  with  the  series  coils,  as  shown  in  Fig.  102,  so  that,  of  the  total 
current  Ii,  only  a  fixed  portion  passes  through  the  series  field  coils. 


FIG.    102. — Series  shunt    to 
vary  the  series  excitation. 


CHAPTER  XIV 
THEORY  OF  OPERATION  OF  DIRECT -CURRENT  MOTORS 

86.  Driving  Force  of  a  Motor. — An  electric  generator  and  an 
electric  motor  are  identical  in  structure.     The  generator  is  used 
to  transform  mechanical  energy  into  electrical  energy,  while  the 
same  machine  operating  as  a  motor  can  be  used  to  transform  elec- 
trical energy  into  mechanical  energy. 

If  a  voltage  is  applied  at  the  terminals  of  the  machine  in 
diagram  B,  Fig.  103,  so  as  to  send  current  through  the  armature 
conductors  in  the  direction  shown,  then,  since  these  conductors 
are  carrying  current  and  are  in  a  magnetic  field,  they  are  acted  on 
by  forces  all  of  which  act  in  the  same  direction  around  the  shaft 
and  so  cause  the  armature  to  rotate. 

87.  Driving  and  Retarding  Forces  in  Generators  and  Motors.— 
The  generator  in  diagram  A,  Fig.  103,  driven  by  an  engine  in  the 


FIG.  103. — Driving  and  retarding  forces  in  a  generator  and  in  a  motor. 

direction  shown,  supplies  electric  power  to  a  circuit,  and  current 
flows  through  the  armature  conductors  in  the  direction  indicated 
by  the  crosses  and  dots.  There  is  a  force  exerted  on  these  conduc- 
tors in  as  much  as  they  are  carrying  current  in  a  magnetic  field, 
which  force  is  opposed  to  the  direction  of  motion,  see  page  13, 
and  the  larger  the  current  the  greater  is  this  retarding  force.  To 
keep  the  generator  running,  the  driving  force  of  the  engine  must 

78 


ART.  88]  OPERATION  OF  DIRECT-CURRENT  MOTORS          79 

be  great  enough  to  overcome  this  retarding  force  and  also  to  over- 
come the  friction  force  of  the  machine. 

The  same  machine  operating  as  a  motor  is  shown  in  diagram  B. 
A  voltage  applied  at  the  motor  terminals  from  some  external 
source  forces  current  through  the  armature  conductors  in  the  di- 
rection shown  and,  since  these  conductors  are  carrying  current  in  a 
magnetic  field,  they  are  acted  on  by  forces  which  cause  the  arma- 
ture to  rotate  in  a  direction  that  may  be  determined  by  the  left- 
hand  rule,  page  7.  Now  the  conductors,  rotating  with  the 
armature,  cut  lines  of  force,  and  an  e.m.f.  is  generated  in  the  wind- 
ing in  exactjy  the  same  way  as  if  the  machine  was  driven  by  an 
engine.  This  e.m.f.  acts  in  the  same  direction  as  in  diagram  A 
since  the  machines  have  the  same  polarity  and  rotate  in  the  same 
direction.  This  generated  e.m.f.  is  therefore  opposed  to  the 
current  in  the  conductors  and  opposed  to  the  applied  e.m.f., 
for  which  reason  it  is  called  the  back  or  counter  e.m.f.  of  the 
motor. 

In  the  case  of  both  a  generator  and  a  motor,  there  is  a  force 
acting  on  the  conductors  of  the  armature  in  as  much  as  they  are 
carrying  current  and  are  in  a  magnetic  field.  This  is  the  driving 
force  in  the  case  of  a  motor  and  the  retarding  force  in  the  case  of  a 
generator.  There  is  also  an  e.m.f.  generated  in  the  armature  of 
each  machine  in  as  much  as  it  is  rotating  in  a  magnetic  field. 
This  e.m.f.  acts  in  the  direction  of  the  current  flow  in  the  case  of 
a  generator  but  opposes  the  current  flow  in  the  case  of  a  motor. 

In  order  that  current  may  flow  through  a  motor  armature,  the 
applied  e.m.f.  Ea  must  be  greater  than  the  back  e.m.f.  Eb  and 

Ea  =  Eb  +  IaRa 

where  Ea  is  the  applied  voltage 
Eb  is  the  back  e.m.f. 

IoRa  is  the  voltage  required  to  force  the  armature  current 
I a  through  the  armature  resistance  Ra  and  is  called  the  arma- 
ture resistance  drop. 

In  the  above  equation  it  is  most  important  to  note  that,  of  the 
applied  voltage  Ea,  the  part  which  forces  the  current  through  the 
resistance  of  the  armature  is  IaRa  and  seldom  exceeds  5  per  cent, 
of  Ea]  the  remaining  part  of  the  applied  voltage  is  required  to 
overcome  Eb,  the  back  generated  voltage  of  the  machine. 

88.  The  Back  E.M.F. — The  existence  of  the  back  e.m.f.  may 
readily  be  shown  by  experiment.  If  for  example  a  motor,  con- 


80         PRINCIPLES  OF  ELECTRICAL  ENGINEERING     [CHAP.XIV 

nected  as  shown  in  Fig.  104,  is  driving  a  flywheel,  and  the  switch 
S  is  suddenly  opened  so  as  to  disconnect  the  armature  from  the 
power  mains,  the  flywheel  will  keep  the  machine  running,  but  the 
ammeter  reading  will  become  zero  and  the  voltmeter  reading  will 
drop  suddenly  from  Ea  to  Eb,  the  voltage  generated  by  the  rotat- 
ing armature,  and  will  then  drop  slowly  to  zero  as  the  motor  comes 
to  rest. 


xAm  meter 

—On — 


,_____ 
)    O Voltmeter 


s 
FIG.  104. — Experimental  determination  of  the  back  e.m.f. 

Example:  Let  Ea,  the  applied  voltage,  be  110,  Ia  the  armature  current, 
100  amp.,  and  Ra  the  armature  resistance,  0.04  ohms.  The  voltmeter 
reading  will  be  110  volts  while  switch  S  is  closed  but  will  drop  suddenly  to 
110  -  (100  X  0.04)  =  106  volts  at  the  instant  the  switch  is  opened,  and  will 
then  drop  slowly  to  zero  as  the  motor  slows  down. 

89.  Theory  of  Motor  Operation. — The  power  taken  by  a  motor 
from  the  mains  changes  automatically  to  suit  the  mechanical 
load.  Consider  the  case  of  a  motor  connected  as  shown  in  Fig. 
105,  the  applied  voltage,  the  exciting  current  //,  and  the  magnetic 
flux  per  pole  being  constant.  If  the  motor  is  at  standstill  and 
the  switch  S  is  closed,  a  large  current  Ia  =  Ea/Ra  will  flow 
through  the  armature,  the  back  voltage  Eb  being  zero  since  the 
armature  conductors  are  not  cutting  lines  of  force.  The  arma- 
ture conductors  carrying  current,  being  in  a  magnetic  field,  are 
acted  on  by  forces  which  overcome  the  resisting  forces  of  friction 
and  of  the  load  and  cause  the  motor  to  rotate.  As  the  motor 
increases  in  speed,  the  back  e.m.f.  Eb  also  increases  since  it  is  pro- 
portional to  the  rate  at  which  the  armature  conductors  cut  lines  of 
force,  and  therefore  the  current  Ia  =  (Ea  —  Eb)/Ra,  see  page 
79,  decreases.  The  motor  will  stop  accelerating  when  this  cur- 
rent has  dropped  to  such  a  value  that  the  total  force  developed  is 
just  sufficient  to  overcome  the  retarding  force. 

If  now  the  load  on  the  motor  is  increased,  the  driving  force  due 
to  the  armature  current  is  not  sufficient  to  overcome  the  increased 
resisting  force  and  the  motor  must  slow  down.  As  the  speed 


ART.  89.]          OPERATION  OF  DIRECT-CURRENT  MOTORS 


81 


decreases,  however,  the  back  e.m.f.  Eb  also  decreases  and  allows  a 
larger  current  to  flow  through  the  armature,  since  Ia  =  (Ea  - 
Eb)/Ra.  The  motor  finally  settles  down  to  such  a  speed  that  the 
increased  current  in  the  armature  again  produces  a  driving  force 
which  is  just  sufficient  to  overcome  the  increased  retarding  force. 
If  the  load  on  the  motor  is  decreased,  the  driving  force  due  to 
the  armature  current  is  more  than  sufficient  to  overcome  the 
decreased  resisting  force  and  the  motor  must  accelerate.  As  it 
increases  in  speed,  however,  the  back  e.m.f.  Eb  also  increases  and 
causes  the  armature  current  Ia  to  decrease.  The  motor  stops 
accelerating  and  the  speed  and 
armature  current  remain  con- 
stant when  the  driving  force 
due  to  the  current  has  dropped 
to  such  a  value  that  it  is  just 
sufficient  to  overcome  the  de- 
creased retarding  force.  The 
electrical  power  taken  by  the 
motor  from  the  mains  there- 
fore changes  automatically  to 
suit  the  mechanical  load  on  the 
motor.  The  back  e.m.f.  of  the 

motor  regulates  the  flow  of  current  in  the  same  way  as  the  gover- 
nor regulates  the  flow  of  steam  in  a  steam  engine. 

A  110-volt  direct-current  motor,  connected  to  the  mains  as  shown  in  Fig.  105 
delivers  10  h.p.  If  the  efficiency  is  88  per  cent.,  the  exciting  current 
is  2  amp.  and  the  armature  resistance  is  0.08  ohms  find: 

a.  The  motor  input 

6.  The  current  taken  from  the  mains 

c.  The  armature  current 

d.  The  back  e.m.f. 

a.  the  motor  output  =  10  h.p. 

output 
the   motor  input  =   ~~^,~r 


pIG 


efficiency 
=  -^  =  11.35   h.p. 


b.  1 1,  the  current  from  the  mains 


=  11.35  X  746  =  8480  watts 
watts     input 


applied  voltage 
8480 


110 


=  77  amp. 


82  PRINCIPLES  OF  ELECTRICAL  ENGINEERING    [CHAP,  xiv 

c.  the  armature  current  =  the  total  current  —  the  exciting  current 

=  77  -  2  =  75  amp. 

d.  the  applied  voltage  =110 
the  voltage  to  overcome  the  resistance  drop  =  IaRa 

=  75  X  0.08 

=  6  volts 

the  back  e.m.f.  Eb  =  Ea  -  IaRa 
=  110-6 
=  104  volts. 

90.  Speed  and  Torque  Formulae. — The  force  on  a  conductor 
carrying  current  in  a  magnetic  field  is  proportional  to  the  current 
and  to  the  strength  of  the  magnetic  field,  see  page  7,  so  that  the 
torque  developed  by  a  given  motor  may  be  expressed  by  the 
equation 

T  =  a  const.  X  <£  X  70 

where   T  is  the  torque  in  Ib.  at  1  ft.  radius,  or  in  kg.  at  1m.  radius 
0  is  the  flux  per  pole  of  the  motor 
I a  is  the  armature  current 

the  constant  depends  on  the  construction  of  the  machine  and 
on  the  units  chosen,  but  with  its  actual  value  we  are  not 
concerned. 

When  a  motor  is  running,  the  back  e.m.f.  is  always  less  than 
the  applied  e.m.f.  by  IaRa  the  armature  resistance  drop,  see  page 
79,  so  that 

Eb   =   Ea  —   IaRa 

Now  Eb  is  generated  in  the  motor  armature  because  the  con- 
ductors are  cutting  lines  of  force,  see  page  79,  so  that,  in  a  given 
machine,  Eb  is  proportional  to  the  flux  per  pole  and  to  the  speed 
dr 

Eb  =  k<f>  r.p.m.  where  k  is  a  constant 
and  Eb  =  Ea  —  IaRa  as  shown  above 

ET 

therefore    r.p.m.  =  T~ 

=  a  const.  X  (E°  ~  ImRm) 

9 

where  r.p.'m.  is  the  motor  speed  in  revolutions  per  minute 
Ea  .     is  the  voltage  applied  at  the  motor  terminals 
IaRa    is  the  armature  resistance  drop. 
(j)         is  the  flux  per  pole  of  the  motor. 


ART.  92] 


OPERATION  OF  DIRECT-CURRENT  MOTORS 


83 


These  speed  and  torque  formulae  will  be  used  in  the  next 
chapter  for  the  determination  of  the  characteristics  of  different 
types  of  motors. 

91.  Improvement  of  Commutation  by  Shifting  of  the  Brushes.— 
In  a  direct-current  generator  the  brushes  are  shifted  from  the 
neutral  in  the  direction  of  motion  so  that  the  coil  in  which  the 
current  is  being  reversed  is  in  what  has  been  called  a  reversing 
field,  see  page  65;  in  the  case  of  the  generator  shown  in  diagram 
A  Fig.  106,  this  reversing  field  is  under  the  tip  of  the  north  pole. 

The  same  machine  operating  as  a  motor  is  shown  in  diagram  B, 
the  direction  of  motion  and  the  polarity  of  the  poles  being 
unchanged,  while  the  direction  of  the  current  is  reversed  in  order 
to  make  the  motor  rotate  in  the  desired  direction.  If  then  the 
reversing  field  for  the  conductor  at  brush  B  of  the  generator 


A  B 

Generator  Motor 

FIG.  106. — Armature  magnetic  field  in  a  generator  and  in  a  motor. 

is  under  tip  of  the  N  pole,  that  for  the  conductor  at  the  same  brush 
of  the  motor  must  be  under  the  tip  of  the  S  pole  since  the  motor  is 
running  in  the  same  direction  as  the  generator  but  with  a  reversed 
current.  From  this  result  the  rule  is  obtained  that  in  a  generator 
the  brushes  should  be  shifted  forward  in  the  direction  of  motion 
whereas  in  a  motor  they  should  be  shifted  backward  against  the 
direction  of  motion. 

92.  Armature  Reaction  in  Generators  and  Motors. — In  Fig. 
106,  which  shows  a  generator  and  a  motor  respectively  with  the 
brushes  shifted  so  as  to  improve  commutation,  the  distribution  of 
magnetic  flux  due  to  the  armature  acting  alone  is  as  shown  by  the 
lines  of  force.  The  armature  field  acts  in  the  direction  oz  and 
may  be  considered  as  the  resultant  of  a  cross  magnetizing  com- 
ponent in  the  direction  oy  and  of  a  demagnetizing  component  in 


84         PRINCIPLES  OF  ELECTRICAL  ENGINEERING     [CHAP,  xrv 

the  direction  ox,  see  page  68,  and,  in  the  case  of  both  the  genera- 
tor and  the  motor,  the  most  important  effects  of  the  reaction  of 
the  armature  field  on  that  due  to  the  exciting  current  in  the  field 
coils  are,  that  the  demagnetizing  effect  reduces  the  flux  per  pole, 
while  the  cross-magnetizing  effect  causes  the  flux  density  to 
decrease  under  the  pole  tips  toward  which  the  brushes  have  been 
shifted,  a  condition  which  tends  to  cause  poor  commutation, 
see  page  68. 


CHAPTER  XV 

CHARACTERISTICS  OF  DIRECT-CURRENT  MOTORS 
SHUNT-WOUND  MOTORS 

93.  The  Starting  Torque. — The    shunt    motor    is    connected 
to  the  power  mains  as  shown  diagrammatically  in  Fig.   107. 
The  applied  voltage  Ea  and  the  exciting  current  //  are  constant 
and  are  independent  of  the  armature  current  Ia. 

To  start  such  a  machine,  the  field  coils  are  fully  excited  so 
that  the  magnetic  flux  has  its  normal  value  and  then  the  re-, 
sistance  Rs  is  gradually  decreased  and  current  allowed  to  flow 
through  the  armature.  The  torque  developed  increases  directly 
as  the  armature  current  is  increased  and  the  motor  will  start  to 
rotate  when  the  current  has  such  a  value  that  the  torque  devel- 
oped is  large  enough  to  overcome  the  resisting  torque  of  friction 
and  of  the  load. 

The  torque  developed,  being  equal  to  &<£/„,  see  page  82, 
depends  only  on  the  flux  per  pole  and  on  the  armature  current 
and,  since  the  exciting  current  and  therefore  the  flux  per  pole  are 
constant,  full -load  current  in  the  machine  produces  the  same 
torque  at  starting  as  when  the  motor  is  running  at  full-load  and 
normal  speed,  or  full-load  torque  is  developed  with  full-load 
current;  similarly  twice  full-load  torque  is  developed  with  twice 
full-load  current  in  the  armature. 

94.  The  Starting  Resistance.— If  a  motor  armature  at  stand- 
still were  connected  directly  to  the  power  mains  then,  since  its 
resistance  is  small,  a  large  current  would  flow  through  the  armature 
and  burn  the  windings  and  the  brushes.     To  limit  the  starting 
current,  a  starting  resistance  must  be  inserted  in  series  with  the 
armature  as  shown  in  Fig.  107  and,  if  full-load  torque  is  required 
at  starting,  this  resistance  must  limit  the  current  to  its  normal 
full-load  value. 

As  soon  as  the  armature  begins  to  rotate,  a  back  e.m.f.  is 
generated  in  it  which  tends  to  make  the  current  decrease  since 

/  Tjl  Tjl     \ 

I* =  fW oT  but,    to    maintain    full-load    torque    until    the 

(jKa  -f-  rCs) 

85 


86 


PRINCIPLES  OF  ELECTRICAL  ENGINEERING      [CHAP,  xv 


motor  is  up  to  speed,  full-load  current  must  be  maintained  in 
the  armature,  so  that  the  starting  resistance  must  gradually  be 
decreased  as  the  motor  comes  up  to  speed  and  the  back  e.m.f. 
increases. 

A  10-h.p.,  110  volt,  direct-current  shunt  motor  has  an  efficiency  of  88 
per  cent.,  an  exciting  current  of  2  amp.  and  an  armature  resistance  of  0.08 
ohms  find: 

a.  The  starting  resistance  required  for  full-load  torque 
6.  The  starting  current  if  no  starting  resistance  was  used 

output 
a.  the  motor  input  =  -~— 

efficiency 


10 

0.88 


=  11.35  h.p. 


=  11.35  X  746  =  8480  watts 

watts  input 

the  current  from  the  mains  = r^3       ii " 

applied  voltage 

8480       _ 

-116  =7 


la 


FIG.  107. — Connec- 
tions of  a  shunt  motor 
during  starting. 


FIG.  108.— Starter  for 
a  shunt  motor. 


FIG.  109.— Starter 
with  a  no-voltage  re- 
lease. 


the  armature  current  =  the  total  current  —  the  exciting  current 
=  77  -  2  =  75  amp. 

applied  voltage 
the  total  resistance  at  starting  =  y  „  r 

full-load  armature  current 

=  -^g-  =  1.47  ohms. 

the  starting  resistance  =  the  total  resistance  —  the  armature  resistance 
=  1.47  -  0.08 
=  1.39  ohms 


ART.  95]  DIRECT-CURRENT  MOTORS  87 

6.  the  starting  current  if  no  starting  resistance  is  used 

applied  voltage 
.'    armature  resistance 

-  t).o°8  •  138°  amP- 

=  18.4  times  full-load  current,  which  would  burn  up 
the  winding. 

95.  Motor  Starter. — A  starter  which  may  be  used  to  perform 
the  operations  described  above  is  shown  diagrammatically  in 
Fig.  108.     When  the  handle  A,  which  is  made  of  metal,  is  moved 
into  position  Ai,  the  field  coils  are  fully  excited  while  the  arma- 
ture and  the-  whole  starting  resistance  are  put  in  series  across  the 
power  mains.     As  the  handle  is  gradually  moved  over  to  position 
A  2,  the  starting  resistance  is  gradually  cut  out  of  the  armature 
circuit,  but  the  current  7/  in  the  field  coils  remains  practically 
unchanged  since  the  starting  resistance  Rs  is  small  compared 
with  Rf,  the  resistance  of  the  field  coils;  in  the  above  problem,  for 

Tjl  110 

example,  Rs  =  1.39  ohms  while  Rf  =  -j-  =    ^  =  55  ohms. 

The  starting  handle  must  not  be  moved  over  too  rapidly,  or  the 
starting  resistance  will  be  cut  out  before  the  speed  and  therefore 
the  back  e.m.f .  have  time  to  increase  and  limit  the  current.  The 
handle  however  must  not  be  left  on  one  of  the  intermediate 
notches  between  A\  and  A 2  because,  in  order  to  keep  down  the 
cost  of  the  starting  resistance,  it  is  made  small  and  will  not  carry 
full-load  current  without  injurious  heating  for  more  than  about 
15  sec. 

96.  No-voltage  Release. — Suppose  that  a  motor  is  running  at 
normal  speed  and  that  the  power  supply  is  interrupted  due  to  some 
trouble  in  the  power  house  or  in  the  line,  the  motor  will  stop,  but 

.the  starting  handle  will  remain  in  the  running  position.  If 
the  power  supply  is  now  re-established,  the  armature  will  be  at 
standstill  and  there  will  be  no  starting  resistance  in  series  with  it 
to  limit  the  current.  To  take  care  of  such  a  contingency  the 
starter  is  changed  by  the  addition  of  what  is  called  the  no-voltage 
release.  A  starter  with  this  attachment  is  shown  diagrammat- 
ically in  Fig.  109.  The  starting  handle  is  moved  from  the  starting 
to  the  running  position  against  the  tension  of  the  spring  S  and  is 
held  in  the  running  position  by  the  electromagnet  M .  If  the 
power  supply  is  now  interrupted,  the  exciting  current  will 
decrease,  the  magnet  M  will  not  be  able  to  hold  the  handle 


88 


PRINCIPLES  OF  ELECTRICAL  ENGINEERING       [CHAP.XV 


against  the  pull  of  the  spring,  and  the  handle  will  be  pulled  back 
to  the  starting  position.  The  magnet  M  is  generally  so  designed 
that  it  will  release  the  starting  handle  should  the  applied  voltage 
drop  below  30  per  cent,  of  its  normal  value.  Such  starters  are 
described  more  fully  in  Chapter  19,  page  114. 

97.  Load  Characteristics. — The  characteristic  curves  of  a  motor 
show  how  the  torque  and  the  speed  vary  with  the  armature 
current,  the  applied  voltage  being  constant.  These  curves  may 
readily  be  determined  from  the  formulae: 


Armature  Current 

FIG.  110. — Characteristic  curves  of  a  shunt  motor. 


torque  =  k<j>I0 

7  (Ea 
r.p.m.  =  k\— 


IaRa) 


where  Ea  is  the  applied  voltage 

I a  is  the  armature  current  in  amperes 
Ra  is  the  armature  resistance  in  ohms 
IaRa,  the  armature   resistance  drop,  seldom  exceeds  5  per 

cent,  of  Ea  when  the  motor  is  carrying  full-load 
0  is  the  flux  per  pole 
k  and  ki  are  constants 

In  the  case  of  the  shunt  motor,  see  Fig.  110,  the  applied  voltage 
Ea  and  the  exciting  current  //  are  constant  and  so  also  is  the  flux 
per  pole,  the  effect  of  armature  reaction  being  neglected,  then: 

torque  =  k<f>Ia 

=?  a  const.  X  Ia 


r.p.m. 


fcr 


=  a  const.  (Ea  —  IaRa) 


ART.  98] 


DIRECT-CURRENT  MOTORS 


89 


The  curves  corresponding  to  these  equations  are  shown  in 
Fig.  110.  The  full-load  speed  is  less  than  that  at  no-load  by 
about  5  per  cent,  since  the  back  e.m.f.  Eb  has  to  drop  this 
amount  in  order  that  full-load  current  may  flow  through  the 
armature.  • 

98.  Effect  of  Armature  Reaction  on  the  Speed. — When  the 
effect  of  armature  reaction  is  neglected,  the  speed  characteristic 
of  a  shunt  motor  is  as  shown  in  Fig.  110;  the  drop  in  speed  seldom 
exceeds  5  per  cent,  at  full-load.     When  the  brushes  are  shifted 
backward  from  the  neutral  so  as  to 'improve  commutation,  arma- 
ture reaction  causes  the  flux  per  pole  to  decrease  as  the  load  in- 
creases, see  page  84,  so  that  the  speed,  being  equal  to  k(Ea  - 
IaRa)/<t>  remains  approximately  constant  from  no -load  to  full- 
load,  since  the  decrease  in  the  value  of  (Ea  —  IaRa)  is  compen- 
sated for  by  the  decrease  in  the  value  of  </>. 

Shunt  motors  are  suited  for  constant-speed  work  such  as  the 
driving  of  line  shafts  and  wood -working  machinery. 

99.  Variable   speed  operation  can  best  be   investigated   by 
means  of  the  equation  r.p.m.  =  k(Ea  —  IaRa)/4>,  see  page  82.- 


MMAMAA 


FIG.  111. — Insert  resis-  FIG.  112. — Insert  resistanc  in  the 
tance  in  the  field  circuit  to  armature  circuit  to  decrease  the 
increase  the  speed.  speed. 

Methods  of  adjusting  the  speed  of  a  shunt  motor. 

To  increase  the  speed,  0  the  flux  per  pole  must  be  reduced  by  in- 
serting a  resistance  in  series  with  the  field  coils  as  in  Fig.  111. 
To  decrease  the  speed  below  the  value  which  it  has  when  the  flux 
per  pole  is  a  maximum,  the  voltage  Ea  applied  to  the  motor 
terminals  must  be  decreased  by  inserting  a  resistance  in 
series  with  the  armature  as  shown  in  Fig.  112;  this  resistance  must 
be  able  to  carry  the  full-load  current  without  injury  so  that  the 
starting  resistance  must  not  be  used  since  it  is  designed  for  start- 
ing duty  only,  see  pa*ge  87. 

While  the  formula  shows  that  the  speed  increases  when  the 


90  PRINCIPLES  OF  ELECTRICAL  ENGINEERING      [CHAP,  xv 

flux  0  is  decreased,  it  is  advisable  to  study  more  fully  how  this 
takes  place.  If  the  flux  per  pole  is  suddenly  decreased,  the  back 
e.m.f.  of  the  motor  drops  and  allows  more  current  to  flow  in  the 
armature.  The  increase  in  the  armature  current  is  greater  than 
the  decrease  in  the  flux,  so  that  the  torque  developed  is  greater 
than  necessary  for  the  load  and  the  motor  accelerates.  The 
following  problem  illustrates  this. 

A  10-h.p.,  110  volt,  900  r.p.m.  direct-current  shunt  motor  has  an 
armature  resistance  of  0.08  ohms  and  takes  an  armature  current  of  75 
amp.  at  full  load.  Find: 

a.  The  torque  at  full-load 

b.  The  back  e.m.f.  at  full-load 

If  the  flux  per  pole  is  suddenly  reduced  to  80  per  cent,  of  normal  find: 

c.  The  back  e.m.f.  at  the  instant  the  flux  is  changed 

d.  The  armature  current  at  the  same  instant 

e.  The  torque  at  the  same  instant 

horse-power  X  33,000 
a.  the  torque  =  - 

27rr.p.m. 

X  33'000  =  58.5  Ib.  at  1  ft.  radius 


6.  the  back  e.m.f.  Eb  =  E  «-  IaRa 

=  110  -  (75  X  0.08)  =  104  volts  at  full-load 

At  the  instant  the  flux  is  reduced 

c.  the  back  e.m.f.  Eb  =  104  X  80/100  =  83.2  volts,  since  0  is  reduced 

d.  the  armature  current  =  (Ea  —  Eb)/Ra 

=  (110  -  83.2)  /0.08 

=  335  amp.  or  4.46  times  full-load  current 

e.  the  torque  =  58.5  X  80/100  X  335/75,  since  it  is  proportional  to  the 

flux  and  to  the  armature  current 
=  209  Ib.  at  1  ft.  radius  or  3.6  times  full-load  torque. 

/.  At  the  instant  the  flux  ,per  pole  is  reduced  to  80  per  cent,  of  its  normal 
value,  the  armature  current  increases  to  4.46  normal  and  the  torque  to  3.6 
times  normal  value.  The  driving  torque  being  then  larger  than  the  retarding 
torque  of  the  load,  the  motor  must  accelerate. 

SERIES-WOUND  MOTORS 

100.  The  Starting  Torque.  —  The  series  motor  is  connected  to 
the  power  mains  as  shown  diagrammatically  in  Fig.  113.  The 
applied  voltage  Ea  is  constant  while  the  field  excitation  increases 
with  the  load. 


ART.  101] 


DIRECT-CURRENT  MOTORS 


91 


The  torque  developed,  being  equal  to  k<j)Ia,  see  page  82, 
increases  directly  with  0  the  flux  per  pole  and  with  Ia  the 
armature  current.  Now  0  increases  with  Ia  since  that  current 
is  also  the  exciting  current  and,  if  the  magnetic  circuit  of  the 
machine  is  not  saturated,  0  is  directly  proportional  to  Ia  and  the 
torque  is  therefore  proportional  to  702.  In  an  actual  motor,  the 
flux  per  pole  does  not  increase  as  rapidly  as  the  exciting  current, 
due  to  saturation  of  the  magnetic  circuit,  but  varies  with  Ia 
as  shown  in  curve  1,  Fig.  113.  Using  this  relation  between 
0  and  I a,  the  relation  between  torque  (/c0/a)  and  Ia  has  been 
determined  and  is  plotted  in  curve  2. 

Full-load  current  in  the  machine  produces  the  same  flux  per 


Torque = k  0 1  a 


f. 


0 

Eftis  Constant 
0  Increases  with  7c 


Armature'Current  la 

FIG.  113 — Characteristic  curves  of  a  series  motor. 

pole  and  therefore  the  same  torque  at  starting  as  when  the  motor 
is  running  at  full-load  and  normal  speed,  or  full-load  torque  is 
developed  with  full-load  current.  Since  the  torque  is  approxi- 
mately proportional  to  I02,  twice  full-load  torque  is  developed 
with  approximately  V2  times  or  1.414  times  full-load  current. 
In  the  case  of  the  shunt  motor,  the  flux  per  pole  is  constant  and 
the  torque  is  directly  proportional  to  Ia,  see  page  85,  so  that 
twice  full-load  torque  requires  twice  full-load  current.  For 
heavy  starting  duty,  therefore,  the  series  motor  is  better  than  the 
shunt  motor  in  that  it  takes  less  starting  current  from  the  line. 
101.  The  Starting  Resistance. — As  in  the  case  of  the  shunt 
motor,  see  page  85,  a  starting  resistance  must  be  inserted 
in  series  with  the  armature  so  as  to  limit  the  starting  current. 


92         PRINCIPLES  OF  ELECTRICAL  ENGINEERING       [CHAP,  xv 

This  resistance  must  be  gradually  decreased  as  the  motor  comes 
up  to  speed. 

102.  Load   Characteristics. — The   characteristic   curves   of   a 
series  motor  may  readily  be  determined  from  the  fundamental 
formulae: 

torque  =  k$Ia 

7    (Ea  -  IaRa) 
r.p.m.  =  K\  — 

9 

where  Ea  is  the  applied  voltage 

I a  is  the  armature  current  in  amperes 
Ra  is  the  combined  resistance  of  the  armature  and 

the  series  field  coils 

IoRa,  the  armature  and  series  field  drop,  seldom  ex- 
ceeds 7  per  cent,  of  Ea  when  the  motor  is  carry- 
ing full-load 
</>    is  the  flux  per  pole 
k  and  ki   are  constants 

In  the  case  of  the  series  motor,  the  applied  voltage  Ea  is 
constant,  while  the  flux  per  pole  varies  with  Ia  as  shown  in  curve 
1,  Fig.  113.  The  relation  between  torque  (fc070)  and  arma- 
ture current  is  plotted  in  curve  2,  while  curve  3  shows  the  rela- 
tion between  r.p.m.  (  ki—  '  }  and  the  armature  current. 

It  is  important  to  note  that,  as  the  load  and  therefore  the 
armature  current  decrease,  the  flux  per  pole  decreases  and  the 
machine  must  speed  up  to  give  the  required  back  e.m.f .  At  light 
loads  the  speed  becomes  dangerously  high  and  for  this  reason  a 
series  motor  should  always  be  geared  or  direct  connected  to  the 
load.  If  a  series  motor  were  belted  to  the  load  and  the  belt  broke 
or  slipped  off,  then  the  motor  would  run  away  and  would  prob- 
ably burst. 

Series  motors  are  suited  for  crane  work  because  they  develop 
a  large  starting  torque,  slow  down  when  a  heavy  weight  is  being 
lifted  and  speed  up  with  light  loads.  Crane  motors  are  geared 
to  the  hoisting  drum  and  are  always  under  the  control  of 
the  operator. 

103.  Speed   Adjustment. — The   speed    of   a   series   motor   is 
proportional  to  (Ea  —  /a-Ra)/0,  see  page  82,  so  that,  for  a  given 
current  Ia.  the  speed  may  be  changed  by  altering  Ea  the  applied 
voltage,  or  0  the  flux  per  pole. 


ART.  104]  DIRECT-CURRENT  MOTORS  93 

If  a  resistance  Re  is  inserted  in  series  with  the  armature  as 
shown  in  Fig.  1 14,  then  the  voltage  applied  at  the  motor  terminals 
is  reduced  by  IaRe  and  the  lower  back  e.m.f.  required  is  obtained 
at  a  lower  speed. 

With  constant  applied  voltage  and  a  given  amature  current 
the  speed  may  be  increased  by  decreasing  the  flux  per  pole.  This 
may  be  done  as  shown  in  Fig.  115  by  shunting  the  series  field 
winding  with  a  resistance  so  that,  of  the  total  current  7a,  only 
part  is  allowed  to  pass  through  the  field  winding.  The  flux  per 
pole  may  also  be  reduced  by  short  circuiting  part  of  the  field  wind- 
ing as  shown  in  Fig.  116,  if  the  switch  S  is  closed,  the  current 


FIG.  114.  FIG.  115.  FIG.  116. 

FIG.  114 — Insert  resistance  in  the  armature  circuit  to  reduce  the  speed. 
FIG.  115. — Shunt  the  field  coils  to  reduce  the  excitation  and  increase  the 
speed. 

FIG.  116 — Short  circuit  part  of  the  field  winding  to  reduce  the  excitation 
and  increase  the  speed. 

Methods  of  adjusting  the  speed  of  a  series  motor. 

passing  through  the  machine  is  not  changed  so  long  as  the  load  is 
kept  constant,  but  the  exciting  ampere  turns  are  reduced  and  so 
therefore  is  the  flux  per  pole. 


COMPOUND  MOTORS 

104.  The  compound  motor  is  a  compromise  between  the  shunt 
and  the  series  motor  and  is  connected  to  the  power  mains  as 
shown  diagrammatically  in  Fig.  117.  The  applied  voltage  Ea  is 
constant  and  so  also  is  the  shunt  current  //,  but  the  current  in 
the  series  field  coils  increases  with  the  load,  so  that  the  flux  per 
pole  increases  with  the  load  but  not  so  rapidly  as  in  the  series 
motor. 

If  a  shunt  and  a  compound  motor  have  duplicate  armatures 
and  the  same  excitation  at  full-load,  then  at  this  load  they  will 


94         PRINCIPLES  OF  ELECTRICAL  ENGINEERING       [CHAP,  xv 
develop  the  same  torque  and  run  at  the  same  speed  since 

torque  =  kr'Ja 

.     (Ea-IaRa) 

r.p.m.  =  fci- — -— 

For  loads  greater  than  full-load,  the  flux  per  pole  of  the  shunt 
motor  is  unchanged  while  that  of  the  compound  motor  is  in- 
creased due  to  the  series  field  coils,  therefore  the  compound  motor 
has  the  greater  torque  but  the  lower  speed.  For  loads  less  than  full- 
load  on  the  other  hand,  the  flux  per  pole  of  the  com  pound  motor  is 
less  than  that  of  the  shunt  motor  due  to  the  decrease  of  the 
current  in  the  series  field  coils,  so  that  the  torque  is  less  and  the 
speed  is  greater  than  in  the  shunt  machine. 

The  speed  and  torque  characteristics  of  a  compound  motor 


Armature  Current 


FIG.  117.  —  Characteristic  curves  of  compound  motors. 

are  shown  in  Fig.  117.  Unlike  the  series  motor,  the  compound 
motor  has  a  safe  maximum  speed  at  no-load  and  so  cannot  run 
away  on  light  loads.  The  speed  of  a  compound  motor  may  be 
decreased  below  normal  by  means  of  a  resistance  inserted  in  the 
armature  circuit,  and  increased  above  normal  by  means  of  a 
resistance  in  the  field  coil  circuit. 

Compound  motors  are  suitable  for  driving  such  machines  as  rock 
crushers  which  may  have  to  be  started  up  full  of  rock,  because 
they  develop  the  large  starting  torque  with  a  smaller  current  than 
the  shunt  motor,  while  they  drop  in  speed  as  the  load  comes  on 
and  thereby  allow  a  flywhe'el  connected  to  the  shaft  to  take  the 
peak  of  the  load. 


CHAPTER  XVI 
LOSSES,  EFFICIENCY  AND  HEATING 

105.  Mechanical  Losses  in  Electrical  Machinery. — In  order  to 
keep  the  armature  of  an  electrical  machine  rotating,  power  is 
required  to  overcome  the  windage  or  air  friction,  the  bearing 
friction,  and  the  friction  of  the  brushes  on  the  commutator.     This 
power  is  not  available  for  useful  work  and  is  called  the  mechan- 
ical loss  in  the  machine. 

In  a  given  machine  this  loss  increases  with  the  speed,  but  at  a 
given  speed  it  is  practically  independent  of  the  load. 

106.  Copper  Losses. — If  Ra  is  the  resistance  of  the  armature 
circuit,  including  the  armature  winding,  the  brush  contacts,  and 
the  series  field  coils  then,  to  force  a  current  Ja  through  this  circuit, 
a  voltage  ea  =  IaRa  is  required.     This  armature  circuit  drop  at 
full-load  seldom  exceeds  5  per  cent,  of  Et,  the  terminal  voltage. 

The  power  expended  in  overcoming  this  vol- 
tage drop  is  equal  to  eala  =  Ia2Ra  watts  and, 
since  this  power  is  not  usefully  employed,  it 
is  called  the  copper  loss  in  the  armature  circuit. 

If  again,  Rf  is  the  resistance  of  the  shunt 
field  coil  circuit,  Fig.  118,  and  //  is  the  shunt  ~ 

current,  then  the  power  expended  in  exciting 
the  machine  is  equal  to  I/2R/  watts  where  //,  which   is   equal 
to  Et/Rf}  seldom  exceeds  5  per  cent,  of  the  current  in  the  ar- 
mature of  the  machine. 

107.  Hysteresis  Loss. — Fig.  119  shows  an  armature  which  is 
rotating  in  a  two -pole  magnetic  field.     If  we  consider  a  small 
block  of  iron  ab  then,  when  it  is  under  the  N  pole  as  shown,  lines  of 
force  pass  through  it  from  a  to  b;  half  a  revolution  later  the  same 
piece  of  iron  is  under  the  S  pole  and  the  lines  of  force  then  pass 
through  it  from  b  to  a  so  that  the  magnetism  in  the  iron  is  re- 
versed.    To  continually  reverse  the  molecular  magnets   of  the 
iron  in  the  armature  an  amount  of  power  is  required  which  is  called 
the  hysteresis  loss  in  the  machine,  see  page  36. 

The  hysteresis  loss  increases  with  the  number  of  reversals  per 

95 


96 


PRINCIPLES  OF  ELECTRICAL  ENGINEERING     [CHAP,  xvi 


second,  that  is  with  the  speed,  it  also  increases  with  the  flux 
density. 

108.  Eddy  Current  Loss. — If  the  armature  A,  Fig.  119,  were 
made  of  a  solid  block  of  iron  then,  as  it  rotated,  e.m.fs.  would  be 
induced  in  the  surface  layers  of  the  iron  and  eddy  currents  would 
flow  through  the  solid  mass,  see  page  55.     The  power  required 

to  maintain  these  currents  is  called  the 
eddy  current  loss  in  the  armature  and 
is  kept  below  3  per  cent,  of  the  arma- 
ture output  by  lamination  of  the  core, 
see  page  58. 

Since  the  e.m.fs.  generated  in  the 
eddy  current  circuits  depend  on  the 
rate  of  cutting  lines  of  force,  the  eddy 
current  loss  will  increase  with  the 
speed  and  with  the  flux  density. 

109.  Stray  Loss. — The  total  loss  in  a  direct-current  machine 
consists  of 

Windage 
Bearing  friction 
Brush  friction 
Hysteresis  loss 
Eddy  current  loss 


FIG.  119.— Reversal  of 
flux  in  the  armature  core  as 
the  armature  rotates. 


Stray  loss 


Mechanical  losses 


Iron  losses 


Copper  loss 

The  term  stray  loss  is  used  in  practice  to  include  the  mechanical 
and  the  iron  losses.  These  losses  do  not  vary  with  the  load  so 
that  they  have  practically  the  same  value  at  no-load  as  at  full- 
load.  The  stray  loss  can  readily  be  measured  at  no-load  by 
running  the  machine  idle  as  a  motor  at  normal  speed  and  normal 
voltage.  The  motor  armature  input  Et!a,  Fig.  120,  is  then  equal 
to  the  mechanical  losses,  the  iron  losses  and  the  small  no-load 
armature  circuit  copper  loss,  which  latter  loss  may  be  neglected 
as  may  be  seen  from  the  following  problem : 

If  a  50-kw.,    110-volt,   shunt   generator  requires  an  armature  current 
of  30  amp.   when   run  as  a  motor    at    no-load    and  normal  speed  and 
voltage,  find  the  stray  loss,  the  resistance  of  the  armature  circuit  being  0.008 
ohms. 
The  armature  input  at  no-load  =  110  X  30  =  3300  watts 

=  stray  loss  +  Ia2Ra 

=  stray  loss  +  (302  X  0.008  =7.2) 
from  which  the  stray  loss  =  3300  —  7  =  3293  watts. 


ART.  110]  LOSSES,  EFFICIENCY  AND  HEATING  97 

In  the  case  of  large  machines,  the  stray  loss  is  generally  de- 
termined by  driving  the  machine  at  normal  voltage  and  speed 
by  means  of  a  small  shunt  motor  the  losses  of  which  are  known. 
The  machines  are  connected  up  as  shown  in  Fig.  121,  the  generator 
is  then  excited  to  give  normal  voltage  E0  and  the  input  to  the 
small  motor  armature  is  determined  from  readings  of  the  voltage 
Em  and  the  current  Ia.  Under  these  conditions  the  input  to 
the  generator  must  be  equal  to  the  windage,  friction  and  iron 
losses  of  the  machine  and  this  input  is  also  equal  to  Emla 
minus  the  motor  losses,  which  latter  losses  are  known. 


r 


Generator 

FIG.    120. — Machine      FIG.  121. — Machine  driven  by  a  small  motor  the 
runs  idle  as  a  motor.  efficiency  of  which  is  known. 

Measurement  of  the  stray  loss  in  a  direct- current  machine. 

110.  The  efficiency  of  a  machine  =  output/input  and  may  be 
calculated  from  test  data  as  in  the  following  example. 

Draw  the  efficiency  curve  for  a  direct-current  flat-compounded  generator 
rated  at  1000  kw.,  600  volts,  given  the  following  data 
stray  loss  =  30  kw. 

Ra  =  the  resistance  of  the  armature  winding,  brush  contacts  and 

series  field  coils  =  0.006  ohms 
Rf  =  the  resistance  of  the  field  coil  circuit  =  20  ohms 

1000  X  1000 

At  tulMoad,  the  load  current  = x^ —        =  1666  amp. 

ouu 

c*f\f\ 

of  which  Ifl  the  shunt  field  current  =  -^  =  30  amp. 

and  Ia,  the  armature  current  =  1696  amp. 

then  the  stray  loss  =  30  kw. 

If*Rj  =  302  X  20  =  18  kw. 
IJRa  =  16962  X  0.006  =  17.2  kw. 
total  loss  =  65.2  kw. 
generator  output  =  1000  kw. 

generator  input  =  1065.2  kw. 
full-load  efficiency  =  94  per  cent. 

The  mechanical  losses,  the  iron  losses  and  the  shunt  field  copper  loss  are  all 
independent  of  the  load  on  the  machine  and  are  often  classed  together  as  the 

7 


98 


PRINCIPLES  OF  ELECTRICAL  ENGINEERING    [CHAP,  xvi 


constant  loss,  the  variable  loss  being  Ia2Ra  the  loss  in  the  armature  circuit, 

500  X  1000 

-so  that  at  half -load,  the  load  current  =  -  =  833  amp. 

oOO 

of  which  the  shunt  field  current  =  30  amp. 
and  the  armature  current  =  863  amp. 

then  the  stray  loss  =  30  kw. 

I;2Rf  =  18  kw. 

IazRa  =  8632  X  0.006  =  4.5  kw. 
total  loss  =  52.5  kw. 

generator  output     =  500  kw. 
generator  input        =  552.5  kw. 
half  load  efficiency  =  90.5  per  cent. 

Other  values  are  worked  out  in  a  similar  way  and  the  results  plotted  as  in 
Fig.  122. 


250  500          750         1000        1250 

Kilowatt  Output 


FIG.  122. — Efficiency  curve  of  a  1000  kw.,  600  volt  direct-current  generator. 
Approximate  values  for  the  full-load  efficiency   of   standard 


generators  and  motors  are: 

Kilowatts 
1 
5 

25 

100 

500 

1000 


Full-load  efficiency 
80  per  cent. 
83 
88 
91 
94 
95 


The  efficiency  of  a  motor  may  be  determined  by  loading  the 
motor  with  a  prony  brake  and  measuring  the  total  electrical  input 
and  the  corresponding  mechanical  output.  Such  a  test  however 
is  rarely  carried  out  except  in  the  case  of  small  motors,  the  effi- 


ART.  Ill] 


LOSSES,  EFFICIENCY  AND  HEATING 


99 


ciency  of  large  machines  is  more  readily  determined  from  meas- 
urements of  the  losses  and  is  worked  up  as  in  the  last  problem. 

111.  Heating  of  Electrical  Machinery. — The  losses  in  an 
electrical  machine  are  transformed  into  heat  which  causes  the 
temperature  of  the  machine  to  rise  above  that  of  the  surrounding 
air.  The  temperature  becomes  stationary  when  the  rate  at  which 
heat  is  generated  is  equal  to  the  rate  at  which  it  is  dissipated. 

The  rate  at  which  heat  is  dissipated  depends  on  the  difference 
between  the  temperature  of  the  machine  and  that  of  the  sur- 
rounding air.  During  the  brief  interval  after  starting  under  load 
this  temperature  difference  is  small,  very  little  heat  is  dissipated 
and  the  temperature  rises  rapidly  as  shown  in  Fig.  123.  As  the 
temperature  increases,  more  of  the  heat  is  dissipated  and  the 
temperature  rises  more  slowly  as  from  b  to  c.  If  the  load  is  now 
taken  off  the  machine,  the  temperature  will  drop  rapidly  at  first 


=  30 




^        — 

c 

"\s 

x^ 

^ 

\ 

s^V 

^ 

rf 

\\ 

^* 

—- 

/ 

^ 

\ 

/ 

a 

^ 

^ 

20   .40   60   80   100   120  140  160  180  200 
Minutes 

FIG.  123. — Heating  curves  of  electrical  machines. 

and  then  more  slowly  as  shown  in  Fig.  123,  the  temperature  drop 
being  more  rapid  when  the  machine  is  rotating  than  when 
stationary  because  of  the  better  ventilation  and  the  better 
convection  of  heat . 

112.  Permissible  Temperature  Rise. — Insulating  materials  lose 
their  mechanical  and  dielectric  strengths  at  high  temperatures, 
for  example,  cotton  becomes  brittle  at  temperatures  greater  than 
85°  C.  and  begins  to  char  at  slightly  higher  temperatures,  so 
that,  when  cotton  is  used  to  insulate  machines,  the  permissible 
rise  of  temperature  is  50°  C.  above  an  air  temperature  of  35°  C. 

Methods  of  insulating  have  been  devised  whereby  cotton,  paper 
and  other  materials  that  become  brittle  and  char  are  not  used, 
fireproof  materials  such  as  enamel,  asbestos  and  mica  being  used 
entirely,  with  such  insulation  higher  temperatures  are  permissible. 


CHAPTER  XVII 
MOTOR  APPLICATIONS 

113.  Limits  of  Output. — If  the  load  on  a  motor  is  increased,  the 
armature  current  and  the  armature  copper  loss  both  increase  and 
the  temperature  of  the  machine  rises.     The  maximum  load  that 
can  be  put  on  a  motor  is  that  with  which  the  temperature  of  the 
machine  reaches  its  safe  maximum  value;  a  greater  load  raises 
the  temperature  to   such  a  value  that  the  insulation  of    the 
machine  is  permanently  injured. 

The  output  of  a  motor  is  often  limited  by  commutation.  When 
interpoles  are  not  supplied,  the  brushes  are  shifted  from  the 
neutral  so  that  commutation  takes  place  in  a  reversing  or  corn- 
mutating  field.  Now  the  effect  of  armature  reaction  is  to  weaken 
this  reversing  field,  see  page  84,  so  that  as  the  armature  current 
increases,  the  reversing  field  becomes  weaker  and  the  motor 
finally  begins  to  spark  at  the  brushes,  after  which  the  load  can  be 
increased  no  further  without  injury  to  the  commutator. 

If  the  commutation  limit  of  output  is  reached  before  the 
heating  limit,  then  interpoles  may  be  supplied  to  improve  commu- 
tation, see  page  65,  and  the  motor  output  may  be  increased  until 
the  temperature  limit  is  reached. 

114.  Open,  Semi-enclosed  and  Totally  Enclosed  Motors. — 
The  cooling  of  a  motor  depends  largely  on  the  circulation  of  air 
through  the  core  and  windings,  so  that  the  frame  should  be  as 
open  as  possible. 

If  chips  and  flying  particles  are  liable  to  get  into  the  windings, 
the  openings  in  the  frame  should  be  covered  with  perforated  sheet 
metal;  the  motor  is  then  said  to  be  semi-enclosed.  This  screen 
throttles  the  air  supply  on  which  the  cooling  of  the  machine 
largely  depends  so  that,  in  order  to  keep  down  the  temperature 
rise,  the  output  of  a  motor  has  to  be  lower  when  semi-enclosed 
than  when  of  the  open  type. 

When  a  motor  has  to  be  totally  enclosed,  as  for  open-air  service, 
the  output  of  the  machine  has  to  be  considerably  reduced  so  as  to 
keep  the  temperature  down  to  a  safe  value  thus: 

100 


ART.  115]  MOTOR  APPLICATIONS  W( 

A  10-h.p.,  220-volt,  600-r.p.m.  motor  with  40°  C.  rise  on  full-load 
as  an  open  machine  can  be  used  to  deliver  9  h.p.  when  semi- 
enclosed  and  about  6  h.p.  when  totally  enclosed  at  the  same 
voltage  and  speed  and  with  the  same  rise  in  temperature. 

115.  Intermittent  Ratings. — It  was  pointed  out  on  page  99  that 
it  takes  a  considerable  time  for  a  motor  to  attain  its  final  tempera- 
ture so  that,  if  a  motor  has  to  be  operated  intermittently  for  short 
periods,  its  output  may  be  considerably  increased. 

With  suitable  windings  a  particular  motor  frame  was  given  the 
following  ratings 

10  h.p.,  220  volts,  600  r.p.m.  continuous  duty 
17  h.p.,  220  volts,  600  r.p.m.  for  1  hour 
22  h.p..  220  volts,  600  r.p.m.  for  1/2  hour 

the  temperature  rise  at  the  end  of  the  specified  time  being  the 
same  in  each  case. 

116.  Effect  of  Speed  on  the  Cost  of  a  Motor. — For  a  given  horse- 
power output,  a  high  speed  motor  is  always  cheaper  than  a  slow 
speed  motor  thus: 

A  10-h.p.,  220-volt,  1200-r.p.m.  shunt  motor  weighs  750  Ib.  and 

costs  $150. 
A  10-h.p.,  220-volt,  600-r.p.m.  shunt  motor  weighs  1250  Ib.  and 

costs  $250. 
A  10-h.p.,  220-volt,  300-r.p.m.  shunt  motor  weighs  1800  Ib.  and 

costs  $350. 
The  reason  for  this  is  as  follows : 

If  a  given  motor  frame  is  supplied  with  two  armatures,  one  of 
which  A  has  half  as  many  conductors  as  B  but  the  conductors 
have  twice  the  cross  section  and  can  therefore  carry  twice  the 
currrent,  then,  when  run  at  the  same  voltage,  armature  A  with  half 
the  conductors  must  run  at  twice  the  speed  of  B  to  give  the  same 
back  e.m.f.,  but  since  armature  A  can  carry  twice  the  current 
of  B  it  can  therefore  deliver  twice  the  output.  Thus  the  armature 
of  a  10  h.p.,  600  r.p.m.  motor  could  be  rewound  to  deliver 
20  h.p.  at  1200  r.p.m.,  15  h.p.  at  900  r.p.m.  or  5  h.p.  at  300 
r.p.m.  and  these  armatures  would  all  have  approximately  the 
same  weight  and  cost. 

117.  Choice  of  Type  of  Motor. — The  characteristic  curves  of  a 
shunt,  a  series,  and  a  compound  motor  are  shown  in  Fig.  124,  the 
motors  having  the  same  torque  and  speed  at  full-load. 

The  shunt  motor  takes  a  current  which  is  proportional  to  the 


102        PRINCIPLES  OF  ELECTRICAL  ENGINEERING    [CHAP.XVII 

torque  required,  and  operates  at  practically  constant  speed  at  all 
loads.  It  must  be  noted  however  that  the  speed  of  such  a  motor 
increases  slowly  as  the  field  coils  heat  up  because  their  resistance 
increases  and  causes  the  excitation  to  decrease.  If  the  motor 
has  been  started  cold,  the  speed  may  increase  10  per  cent,  in  3 
hours  due  to  this  cause. 

The  series  motor  is  the  best  for  heavy  starting  duty  because,  for 
any  torque  greater  than  the  full-load  value,  it  takes  a  smaller 
current  from  the  line  than  either  the  shunt  or  the  compound 
machine.  The  speed  of  the  series  motor  decreases  rapidly  with 
increase  of  load  and  becomes  dangerously  high  at  light 
loads.  For  this  latter  reason,  a  series  motor  should  always  be 
geared  or  direct  connected  to  the  load. 

The  compound  motor  is  a  compromise  between  the  shunt  and 


Armature  Current 


Armature  Current 


FIG.  124. — Characteristic  curves  of  direct  current  motors. 

the  series  motor.  It  is  better  than  the  shunt  motor  for  heavy 
starting  duty  but  not  so  good  as  the  series  motor.  The  speed 
drops  somewhat  with  the  load  but  the  motor  runs  at  a  safe  maxi- 
mum speed  even  at  no-load. 

The  service  for  which  each  type  of  motor  is  suited  can  best  be 
illustrated  by  a  discussion  of  a  few  typical  motor  applications. 

118.  A  line  shaft  should  run  at  practically  constant  speed  at 
all  loads  and  so  is  driven  by  a  shunt  motor.     The  starting  torque 
required  will  seldom  exceed  1.5  times  full-load  torque  and  this 
can  be  obtained  with  1.5  times  full-load  current  in  the  armature, 
which  is  a  reasonable  starting  overload. 

119.  Wood-working  machinery  such  as  planers  and  circular 
saws  run   at   practically   constant  speed   and   so  are  suitably 


ART.  120]  MOTOR  APPLICATIONS  103 

driven  by  shunt  motors.  In  the  case  of  heavy  planing  mills,  the 
starting  torque  required  is  sometimes  excessive  due  to  the  inertia 
of  the  machine,  in  which  case  it  may  be  advisable  to  use  a 
compound  motor  because  it  requires  a  smaller  starting  current 
for  the  same  torque. 

120.  Reciprocating  pumps,  which  have  to  start  up  against  full 
pressure,  require  a  large  starting  torque,  so  that  although  a  shunt 
motor  is  often  used  for  such  service  yet  a  compound  motor  would 
take  less  starting  current  from  the  line.     A  series  motor  would  be 
suitable  so  far  as  starting  torque  is  concerned,  but  if  the  suction 
pipe  were  to  leak  so  that  the  load  on  the  motor  became  light, 
then  the  motor  would  run  away. 

121.  Traction  Motors. — For  traction  service,  the  torque  re- 
quired to  start  and  accelerate  a  car  is  much  greater  than  that 
required  to  keep  the  car  moving,  so  that  a  series  motor  is  used 
since  it  is  the  best  for  heavy  starting  duty,  see  page  91.     The 
subject  of  traction  is  discussed  more  fully  in  Chapter  40,  page 
322. 

122.  Crane  Motors. — The  characteristics  which  make  the  series 
motor  suitable  for  traction  work  also  make  it  suitable  for  crane 
service.     The  motor  is  able  to  develop  a  large  starting  torque 
without  taking  an  excessive  current  from  the  line;  it  also  oper- 
ates at  a  slow  speed  when  the  load  to  be  lifted  is  heavy  and  runs 
at  a  high  speed  when  the  load  is  light. 

Both  crane  and  traction  motors  are  geared  to  the  load  and 
moreover  are  always  under  the  control  of  the  operator. 

123.  Express  Passenger  Elevators. — An  express  elevator  has  to 
be  accelerated  rapidly,  so  that  a  large  starting  torque  is  required. 
After  the  car  has  moved  through  about  20  ft.,  its  velocity  has 
reached  about  500  ft.  per  min.,  and  has  to  be  kept  constant  at  this 
value.     This  result  is  obtained  by  the  use  of  a  heavily  compounded 
motor,  by  means  of  which  a  large  starting  torque  is  developed 
without  an  excessive  current  being  taken  from  the  line;  when 
acceleration  is  complete,  the  series  winding  is  short  circuited,  and 
the  motor  operates  thereafter  as  a  constant  speed  shunt  machine. 

A  series  motor  would  not  be  suitable  for  such  service  because, 
during  the  rush  hours  when  the  car  is  heavily  loaded  the  motor 
would  slow  down,  whereas  at  times  of  light  load  the  car  would  run 
at  an  excessive  speed,  unless  specially  controlled. 

124.  Shears  and  Punch  Presses. — The  load  curve'of  a  punch 
press  is  shown  in  Fig.  125.     In  order  that  the  peak  load  may  be 


104       PRINCIPLES  OF  ELECTRICAL  ENGINEERING    [CHAP.XVII 


carried  by  the  motor  without  sparking,  a  motor  of  about  15 
h.p.  would  be  required,  which  is  much  greater  than  the 
average  load  of  6.5  h.p.  To  take  the  peak  load  off  the 
motor,  a  flywheel  is  generally  supplied  with  the  pr^ss  and,  in 
order  that  the  flywheel  may  be  effective,  the  speed  of  the  motor 
must  drop  as  the  load  comes  on. 

A  shunt  motor  is  not  suitable  for  such  service  as  it  does  not  drop 
in  speed  and  so  does  not  cause  the  flywheel  to  take  the  load. 

A  series  motor  cannot  be  used  because  it  would  run  away  when 
the  clutch  of  the  press  was  released,  and  would  probably  cause 
the  flywheel  to  burst. 

The  motor  generally  used  on  large  presses  is  compound  wound, 
which  motor  drops  in  speed  as  the  load  comes  on  and  thereby 
causes  the  flywheel  to  give  up  energy,  while  the  maximum 
speed  at  no-load  cannot  exceed  a  safe  value. 


Time 

FIG.  125. — Load  curve  of  a  punch  press. 


FIG.  126. 


A  drooping  speed  characteristic  may  be  obtained  from  a  shunt 
motor  by  connecting  a  resistance  permanently  in  series  with  the 
armature  as  shown  in  Fig.  126,  the  resistance  having  such  a  value 
that  the  voltage  drop  er  at  full-load  is  about  5  per  cent,  of  E. 
When  the  load  on  the  motor  increases,  the  voltage  drop  across 
the  resistance  increases,  that  applied  to  the  motor  terminals 
decreases,  and  the  speed  of  the  motor  drops.  When  the  load  on 
the  motor  decreases,  the  voltage  across  the  motor  increases,  the 
speed  rises,  and  energy  is  again  stored  in  the  flywheel.  The 
resistance  in  the  power  mains  supplying  the  motor  may  often  be 
sufficient  to  produce  this  effect.  The  only  objection  to  the 
method  is  that  an  amount  of  power  =  erla  watts  is  lost  in  the 
control  resistance. 


CHAPTER  XVIII 


ADJUSTABLE  SPEED  OPERATION  OF  DIRECT -CURRENT 

MOTORS 

For  the  driving  of  lathes  and  other  such  machine  tools,  ad- 
justable speed  motors  are  largely  used,  it  is  therefore  necessary  to 
discuss  the  different  methods  of  speed  control  before  the  subject 
of  machine  tool  driving  can  be  profitably  taken  up. 

125.  Speed  Variation  of  Shunt  Motors  by  Armature  Control.— 
The  speed  of  a  motor  may  be  lowered  by  decreasing  the  voltage 

+  77  Amp.  /a=7oAmp.,  Jfl  =  75 


FIG.  127. — Resistance  inserted  in  FIG.  128. — Resistance  inserted  in 

the  armature  circuit  causes  the  the  field  coil  circuit  causes  the 
speed  to  decrease.  speed  to  increase. 

Methods  of  adjusting  the  speed  of  a  direct  current  shunt  motor. 

applied  to  the  motor  terminals.  This  may  be  done  by  connecting  a 
resistance  in  the  armature  circuit  as  shown  in  Fig.  127. 

/  Tfi  j     r>  \ 

The  speed  is  given  by  the  formula  r.p.m.  =  kr       — -— 

where  IaRa  seldom  exceeds  5  per  cent,  of  Ea,  so  that,  to  obtain 
half  speed,  the  applied  voltage  Ea  must  be  reduced  to  about  50  per 
cent,  of  normal,  the  other  50  per  cent,  of  the  line  voltage  being 
absorbed  by  the  resistance  inserted  in  the  circuit;  under  these 
conditions,  the  loss  in  the  resistance,  which  is  equal  to  erla>  is  also 
equal  to  the  armature  input  Eala,  and  the  efficiency  of  the  system 
is  less  than  50  per  cent.  The  actual  efficiency  may  be  figured  out 
as  in  the  following  problem. 

105 


106  PRINCIPLES  OF  ELECTRICAL  ENGINEERING       [CHAP,  xvm 

A  10-h.p.,  110-volt,  900-r.p.m.  shunt  motor  has  an  efficiency  of  88  per  cent., 
an  armature  resistance  of  0.08  ohms  and  a  shunt  field  current  of  2  amp. 
If  the  speed  of  this  motor  is  reduced  to  450  r.p.m.  by  inserting  a  resistance  in 
the  armature  circuit,  the  torque  of  the  load  being  constant,  find  the  motor 
output,  the  armature  current,  the  external  resistance  and  the  overall 
efficiency. 

At  normal  load: 

the  motor  output  =  10  h.p. 

the  motor  input  =  10/0.88      =  11.35  h.p.  =  8480  watts 

the  total  current  =  8480/110  =  77  amp. 

the  shunt  current  =  2  amp. 

the  armature  current  =  75  amp.,  see  page  82 

i  c\  s^  ^  *^  o  on 
the  torque  A  =  58.5  Ib.  at  1  ft.  radius,  see  page  90 

^  /\  TT  /s  yuu 

the  back  e.m.f.  =  Ea  —  IaRa 

=  110  -  (75  X  0.08)  =  104  volts,  see  page  82. 

At  half  speed: 

torque  X  2ir  r.p.m 
The    horsepower    output  =  337)7)0  a     '    since    the    torque 

is  constant,  the  output  is  proportional  to  the  speed  and  is  equal  to  5  h.p. 

The  torque  =  k<j>Ia,  and,  since  the  torque  is  constant  and  so  also  is  the 
excitation,  therefore  /<,,  the  armature  current,  is  the  same  as  at  full  speed  and 
is  equal  to  75  amp. 

The  back  e.m.f.  Eb  is  generated  in  the  armature  due  to  the  cutting  of  lines 
of  force  and  is  equal  to  a  const.  X<£  X  r.p.m.,  see  page  82,  and  since  the  flux 
is  constant,  therefore  Eb  is  proportional  to  the  speed  and  is  equal  to  0.5  X  104 
=  52  volts. 

The  voltage  applied  to  the  motor  =  Eb  +  IaRa 

=  52  +  (75  X  0.08) 
=  58  volts. 

The  voltage  drop  across  the  external  resistance  =  110  —  58 

=  52  volts 

the  current  in  this  resistance  =  75  amp. 
the  resistance  =  52/75  =  0.7  ohjns. 
the  loss  in  the  resistance  =  52  X  75  =  3900  watts, 
the  total  input  =  110  X  (75  +  2),  see  Fig.  127,  =  8500  watts, 
the  motor  output  =  5  hp.  =  5  X  746/1000  =  3.7  kw. 
the  overall  efficiency  =  3.7/8.5  =  44  per  cent. 

Since  the  armature  current  and  therefore  the  armature  copper 
loss  have  the  same  value  at  half  speed  as  at  full  speed,  the  torque 
being  constant,  the  temperature  rise  will  be  the  greater  at  the 
slow  speed  because  of  the  poorer  ventilation. 

From  the  above  problem  it  may  be  seen  that,  when  the  speed  of 
a  motor  is  reduced  by  armature  resistance,  the  output  is  decreased 
and  is  directly  proportional  to  the  speed  while  the  temperature 


ART.  127]  ADJUSTABLE  SPEED _OPERATION  107 

rise,  even  with  this  reduced  output,  is  greater  than  normal 
because  of  the  poorer  ventilation.  The  overall  efficiency  also  is 
exceedingly  low,  the  per  cent,  loss  in  the  resistance  being  ap- 
proximately equal  to  the  per  cent,  reduction  in  speed,  that  is, 
being  50  per  cent,  of  the  total  input  at  half  speed  and  75  per  cent, 
of  the  total  input  at  quarter  speed. 

126.  Speed  Variation  of  Shunt  Motors  by  Field  Control.— 
By  inserting  a  resistance  in  the  field  coil  circuit  of  a  shunt  motor, 
as  in  Fig.  128,  the  excitation  and  therefore  the  magnetic  flux  are 
reduced  and  the  motor  has  to  run  at  a  higher  speed  in  order  to 
generate  the  necessary  back  e.m.f.,  see  page  89. 

When  interpoles  are  not  supplied,  the  brushes  are  shifted  from 
the  neutral  position  so  that  commutation  takes  place  in  a  reversing 
field,  but  if  the  excitation  is  decreased,  then  this  reversing  field  is 
decreased  and  the  commutation  is  impaired;  furthermore,  the 
higher  the  speed,  and  therefore  the  more  rapidly  the  current  in  the 
coils  is  being  commutated,  the  greater  is  the  voltage  of  self  induc- 
tion opposing  the  change  of  current  and  the  greater  the  tendency 
for  sparking  to  take  place  at  the  brushes,  see  page  63.  The  range 
of  speed  variation  is  therefore  limited  by  commutation  and  an 
increase  in  speed  of  about  70  per  cent,  is  about  all  that  can 
generally  be  obtained  by  field  weakening  from  a  standard  motor ; 
the  flux  is  then  reduced  to  1/1.7  =  60  per  cent,  of  its  normal  value. 
When  a  greater  speed  range  is  required,  it  will  generally  be  neces- 
sary to  use  an  interpole  motor. 

When  the  speed  is  controlled  by  a  field  rheostat  the  efficiency 
is  not  impaired  because,  as  shown  in  Fig.  128,  the  control  rheostat 
carries  only  the  small  shunt  current  and  not  the  large  armature 
current. 

127.  Speed  Regulation  of  an  Adjustable  Speed  Shunt  Motor.— 
When  the  speed  of  a  motor  varies  considerably  with  change  of  load 
the  speed  regulation  is  said  to  be  poor.     When  the  speed  is  prac- 
tically constant  at  all  loads  the  speed  regulation  is  said  to  be  good. 

Suppose  that  the  speed  of  a  shunt  motor  has  been  adjusted  by 
means  of  a  resistance  in  the  armature  circuit,  as  shown  in 
Fig.  127,  so  as  to  give  a  definite  speed  at  a  definite  load,  then,  when 
the  load  is  increased,  the  armature  current  Ia  and  the  voltage  er 
will  increase  and  therefore  the  voltage  Ea  will  decrease  and  the 
speed  of  the  motor  will  drop;  the  speed  regulation  is  therefore  poor 
when  armature  resistance  control  is  used. 

If,  for  example,  this  method  of  control  is  used  when  a  forging 


108      PRINCIPLES  OF  ELECTRICAL  ENGINEERING    [CHAP.XVIII 

such  as  that  in  Fig.  129  is  being  turned  in  a  lathe,  then  the 
motor  will  slow  down  when  the  cut  is  deep  and  will  speed  up  when 
the  cut  is  light,  so  that  the  speed  will  be  very  irregular. 

When  the  speed  of  a  shunt  motor  .is  adjusted  by  means  of  a 
resistance  in  the  field  coil  circuit,  as  in  Fig.  128,  the  speed 
regulation  is  good.  The  speed  is  given  by  the  formula  r.p.m.  = 

/  rr     T    r>  \ 

k\—  -  so  that  since  Ea  is  constant,  as  also  is  the  flux  $ 

9 

once  the  field  circuit  resistance  is  adjusted,  therefore  the  drop  in 
speed  between  no-load  and  full-load  will  seldom  exceed  5  per  cent., 

since  IaRa  at  full-load  seldom  exceeds 
5  per  cent,  of  Ea. 

128.  Electric  Drive  for  Lathes  and 
Boring  Mills. — Such  machines  require 
constant  horse-power  at  all  speeds  of 
the   machine  spindle  so  long  as  the 
FIG   129  amount  of  metal  removed  per  minute 

remains  unchanged.     The  motor  must 

therefore  be  large  enough  to  develop  the  necessary  horsepower 
at  the  lowest  operating  speed  without  excessive  heating. 

The  maximum  speed  of  a  given  motor  is  limited  by  centrifugal 
force  or  by  the  speed  of  the  gear,  while  the  minimum  speed  may 
be  as  low  as  desired,  but  when  constant  horsepower  is  required  at 
all  speeds  the  cost  of  the  motor  increases  as  the  minimum  speed 
is  decreased  as  may  be  seen  from  the  following  table: 

A.  A  10-h.p.,  220-volt,  1200-r.p.m.  shunt  motor  weighs  750  Ib. 
and  costs  $150. 

B.  A  10-h.p.,  220-volt,  600/ 1200-r.p.m.  shunt  motor  weighs  1250 
Ib.  and  costs  $250. 

C.  A  10  h.p.,  220-volt,  300/1200-r.p.m.  shunt  motor  weighs  1800 
Ib.  and  costs  $390. 

the  latter  motor  is  necessarily  an  interpole  machine  because  of  the 
large  speed  variation  required,  see  page  107,  and  costs  about  10 
percent,  more  than  a  constant-speed  300-r.p.m.  motor  of  the  same 
output. 

The  cheapest  drive  from  the  point  of  view  of  motor  cost  is  that 
obtained  by  the  use  of  a  constant-speed  motor  such  as  A  in  the 
above  table,  with  change  gears  or  coned  pulleys  to  give  the 
necessary  speed  range.  If  some  of  the  gears  are  eliminated,  and 
a  motor  such  as  B  in  the  above  table  is  used,  which  has  a  speed 
range  of  2  to  1,  then  the  cost  of  the  motor  is  increased  66  per  cent., 


ART.  129] 


ADJUSTABLE  SPEED  OPERATION 


109 


while  for  a  speed  range  of  4  to  1  by  motor  control  the  cost  of  the 
motor  is  2.6  times  greater  than  if  a  constant  speed  motor  with 
change  gears  had  been  used. 

The  speed  range  of  the  motor  may  be  obtained  by  armature 
control,  by  field  control,  or  by  a  combination  of  the  two.  Arma- 
ture control  is  used  as  little  as  possible  because  of  the  low  overall 
efficiency  of  the  method  and  also  because  of  the  poor  speed 
regulation  obtained,  see  pages  105  and  107. 

129.  Multiple  Voltage  Systems. — When  a  large  number  of 
adjustable  speed  motors  are  in  operation  in  a  machine  shop,  the 
three  wire  system  shown  diagrammatically  in  Fig.  130  may  be 
used  with  advantage.  Two  generators  in  the  power  house  are 
connected  in  series  and  three  leads  a,  b  and  c  are  taken  to  each 


220  Volts 


A  B 

FIG.  130. — Multiple  voltage  system. 

adjustable  speed  motor,  the  voltage  between  a  and  c  being  220 
and  between  a  and  b  and  also  between  b  and  c  being  110  volts. 
To  obtain  the  lowest  speed  from  a  motor  operating  on  this 
system,  the  field  coils  are  connected  across  the  220-volt  mains  so 
as  to  give  the  maximum  flux  while  the  armature  is  connected 
across  either  of  the  110-volt  circuits  as  shown  at  A,  Fig.  130. 
The  speed  may  then  be  gradually  increased  by  inserting  resistance 
R  in  the  field  coil  circuit  as  shown  at  B.  When  the  speed  has 
been  doubled  in  this  way,  the  armature  is  then  connected  across 
the  220-volt  mains  and  all  the  resistance  R  is  cut  out  of  the  field 
coil  circuit,  and  the  speed  may  again  be  gradually  increased  by 
once  more  reducing  the  field  excitation  by  means  of  the  resistance 
R  as  shown  at  C.  By  this  means  a  total  speed  range  of  4  to  1  may 
be  obtained  without  the  magnetic  flux  being  reduced  at  any  time 


110    PRINCIPLES  OF  ELECTRICAL  ENGINEERING      [CHAP,  xviu 


to  less  than  half  its  normal  value,  and  the  efficiency  is  high  over 
the  whole  range  of  speed  because  no  resistance  is  inserted  in  the 
armature  circuit  at  any  time. 

Another  multiple  voltage  system  is  shown  diagrammatically 
in  Fig.  131  and  is  in  use  in  a  considerable  number  of  machine 
shops.  The  voltages  available  in  this  case  are  90,  160  and  250 
so  that  a  total  speed  range  of  4  to  1  may  readily  be  obtained  from 
a  standard  motor  since  the  flux  is  never  reduced  below  about  60 
per  cent,  of  ..its  normal  value,  see  page  107.  The  two-voltage 
system  is  to  be  preferred  however  since  110  and  220  are  standard 
voltages  and  still  more  so  because  the  two  voltages  may  readily 
be  obtained  from  a  single  machine  called  a  three-wire  generator, 
which  is  much  cheaper  than  two  single 
voltage  machines  each  of  half  the  output. 
This  three-wire  generator  is  described  on 
page  317. 

130.     Ward     Leonard     System. — One 
method  of  obtaining  a  wide  speed  range, 
without  the  use  of  armature  resistance, 
would  be  to  use  a  separate  generator  with 

,.    each  adjustable  speed  motor  and  to  vary 

FIG.  131. — Multiple  volt-  J 

age  system.  the   excitation   of  the  generator  so  as  to 

vary  the  voltage  applied  to  the  motor  ter- 
minals. Such  a  system,  shown  diagrammatically  in  Fig.  132  is 
called  the  Ward  Leonard  System  after  its  inventor.  The  outfit 
consists  of  a  high-speed  motor  generator  set  supplied  with  each 


T 


Jo 


Eg 


Motor  Generator 
Set 


Adjustable  Speed 
Motor 


FIG.  132. — Ward  Leonard  system. 

adjustable  speed  motor,  the  set  consisting  of  a  high-speed  gener- 
ator G  direct  connected  to  a  motor  M . 

To  obtain  slow  speeds,  the  field  excitation  of  G  is  reduced  so 
as  to  reduce  the  voltage  Eg  which  is  applied  to  the  motor  M\. 
As  the  field  excitation  of  G  is  increased,  the  voltage  Eg  increases 


ART.  131]  ADJUSTABLE  SPEED  OPERATION  111 

and  with  it  the  speed  of  the  motor  MI.  The  motor  M i  can 
readily  be  reversed  by  reversing  the  excitation  of  G,  so  that  the 
whole  control  is  handled  through  a  small  field  circuit  rheostat 
and  the  efficiency  is  comparatively  high. 

The  Ward  Leonard  system  has  been  used  for  printing  press 
work  to  obtain  the  very  low  speeds  required  when  the  paper  is 
being  fed  into  the  machine,  it  has  also  been  used  to  secure  the 
delicate  speed  adjustment  necessary  for  operating  gun  turrets 
in  battleships.  Because  of  the  ease  with  which  the  motor  MI 
can  be  reversed,  this  system  has  been  used  recently  for  the  drive  of 
large  reversing  planers,  so  as  to  eliminate  the  crossed  belts. 

131.  Drive  for  Ventilating  Fans. — For  ventilating  purposes  it 
is  necessary  to  control  the  volume  of  air  delivered,  to  suit  the 
requirements.  This  is  done  either  by  reducing  the  speed  of  the 
fan  or  by  throttling  the  orifice. 

When  a  fan  is  operated  with  a  fixed  orifice,  the  volume  of  air 
delivered  is  proportional  to  the  speed,  the  pressure  is  proportional 
to  the  square  of  the  speed,  and  the  power  required  is  proportional 
to  the  cube  of  the  speed  approximately. 

When  the  volume  of  air  is  reduced  by  throttling  the  discharge, 
then  the  power  taken  is  directly  proportional  to  the  volume 
delivered,  the  speed  of  the  fan  being  constant. 

For  adjustable  speed  operation  the  shunt  motor  is  used  and,  in 
the  case  of  a  fan  drive,  the  speed  reduction  is  obtained  by  the 
armature  resistance  method  of  control  because  of  its  simplicity; 
the  total  loss  in  the  controlling  resistance  being  small  because 
of  the  large  reduction  in  the  load  and  therefore  in  the  armature 
current  as  the  speed  drops.  This  may  be  seen  from  the  following 
example,  which  is  worked  out  by  the  same  method  as  that  on  page 
106: 

The  following  data  is  taken  from  page  106: 

The  output  is  10  h.p.,  110  volts,  900  r.p.m.  and  the  motor  is  shunt  wound, 

the  armature  resistance  =  0.08  ohms 

the  exciting  current  =  2  amp. 

the  full-load  armature  current  =  75  amp. 

the  back  e.m.f.  at  full-load  and  full  speed  =  104  volts. 

If  the  motor  is  driving  a  fan  and  the  speed  is  reduced  to  450  r.p.m.  by 
inserting  a  resistance  in  the  armature  circuit,  find  the  motor  output,  the 
armature  current,  the  external  resistance  and  the  overall  efficiency. 

The  horsepower  output  is  proportional  to  the  cube  of  the  speed  approxi- 
mately and  is  therefore  equal  to  -z  =  1.25  horsepower. 


112       PRINCIPLES  OF  ELECTRICAL  ENGINEERING  [CHAP.XVIII 

horsepower  output  X   33,000 

The  torque  =  —  and  is  therefore  equal  to  full- 

2  IT  r.p.m. 

load  torque  X  ~in  u"'     ^450  =  ^-25  times  full-load  torque. 

The  torque  =  k<j>Ia  and,  since  the  flux  <£  is  constant,  therefore  the  current  is 
proportional  to  the  torque,  so  that  the  armature  current  is  equal  to  0.25  (full- 
load  current)  or  18.8  amp. 
The  back  e.m.f.  at  half  speed  =  104/2  =  52  volts,  see  page  106. 

The  voltage  applied  to  the  motor  =  Eb  +  Ia  Ra 

=  52  +  (18.8  X  0.08)       • 
=  53.5  volts 

The  voltage  drop  across  the  external  resistance  =  110  —  53.5 

=  56.5  volts. 

the  current  in  this  resistance  =  18.8  amp. 

the  resistance  =  56. 5/ 18.8  =  3  ohms 

the  loss  in  the  resistance  =  56.5  X  18.8  =  1060  watts 

the  total  input  =  110  X  (18.8  +  2)  =  2290  watts 

the  motor  output  =  1.25  h.p.  =  1.25  X  746/1000  =  0.93  kw. 

the  overall  efficiency  =  0.93/2.29  =  41  per  cent. 

so  that,  although  the  overall  efficiency  is  still  low,  the  total  loss  in  the 

control  resistance  is  comparatively  small. 

132.  Armature  Resistance  for  Speed  Reduction. — From  the 
two  problems  on  pages  106  and  111  it  may  be  seen  that  the 
resistance  required  to  reduce  the  speed  to  50  per  cent,  of  normal 
may  be  0.7  ohms  or  3  ohms,  the  corresponding  losses  in  these 
resistances  may  be  3.9  kw.  or  1.06  kw.,  and  the  currents  75  amp. 
or  18.8  amp.,  depending  entirely  on  the  kind  of  load. 

If  a  rheostat  built  originally  for  constant  torque  duty  is  used 
with  the  same  motor  for  fan  operation,  it  will  not  have  sufficient 
resistance  to  reduce  the  speed  to  the  desired  value;  if  a  fan  duty 
rheostat  is  used  for  constant  torque  service  it  will  have  to  carry 
a  larger  current  than  it  was  designed  for  and  will  burn  out. 
It  is  therefore  very  essential  when  specifying  armature  rheostats 
to  specify  the  type  of  service  for  which  it  will  be  used. 

133.  Motors  for  Small  Desk  Fans. — These  are  usually  series 
motors  and  are  connected  directly  to  the  line  without  a  starting 
resistance.     When  the  current  is  switched  on,  the  motor  is  at 
standstill  and  its  back  e.m.f.  is  zero,  but  the  growth  of  the  cur- 
rent in  the  machine  is  opposed  by  the  self  induction  of  the  field 
and  armature  windings  which  are  connected  in  series,  while  the 
inertia  of  the  armature  and  fan  are  so  small  that  the  machine  is 
up  to  speed  before  the  current  has  time  to  reach  a  dangerous 
value.     The  load  on  a  fan  increases  as  the  motor  speeds  up  so 
that  the  motor  cannot  run  away. 


ART.  134] 


ADJUSTABLE  SPEED  OPERATION 


113 


134.  Printing  presses  must  be  run  very  slowly  while  being 
made  ready,  that  is,  while  the  web  is  being  threaded,  after  which 
they  must  be  smoothly  accelerated  to  the  desired  running 
speed. 

Large  presses  are  equipped  with  two  shunt  motors,  a  main  driv- 
ing motor  which  is  direct  connected  to  the  driving  shaft,  and  an 
auxiliary  starting  motor  which  is  connected  to  the  driving  shaft 
through  a  reduction  gear  and  an  automatic  clutch. 

While  the  press  is  being  made  ready,  the  auxiliary  motor  alone 
is  connected  to  the  power  mains  and  drives  the  press  at  about 
10  per  cent,  of  normal  speed.  When  the  press  is  ready,  the  main 
motor  is  connected  to  the  power  circuit  and  is  gradually  accele- 
rated, and,  when  the  speed  of  the  press  has  increased  sightly  above 
the  make  ready  speed,  the  au- 
tomatic clutch  is  released  due 
to  centrifugal  force  and  the 
small  motor  is  thereby  dis- 
connected mechanically  from 
the  press;  it  may  then  be  dis- 
connected from  the  power 
mains. 

For  small  presses,  the  aux- 
iliary starting  motor  is  dis- 
pensed with,  the  make  ready 


FIG. 


133. — Connection  for  slow  speed 
operation  of  shunt  motors. 


speed  being  obtained  by  in- 
serting a  resistance  in  the  ar- 
mature circuit.  One  objec- 
tion to  armature  control  is  that  the  speed  regulation  is  poor, 
see  page  107,  and,  when  sufficient  resistance  is  inserted  to  ob- 
tain 10  per  cent,  of  normal  speed,  the  regulation  is  very  poor 
and  the  speed  of  the  press  is  irregular.  To  obtain  low  speeds 
which  are  not  irregular,  the  connection  shown  in  Fig.  133  is  used. 
If  the  current  72  is  large  compared  with  70,  then  a  considerable 
change  in  the  value  of  Ia  will  have  comparatively  little  effect  on 
the  total  current  /i  so  that  the  voltage  er  will  not  be  greatly 
affected,  the  voltage  Ea  will  therefore  remain  approximately  con- 
stant and  the  speed  regulation  will  be  fairly  good.  The  method 
is  not  economical  since  the  current  1 2  does  no  useful  work  but  the 
larger  the  value  of  J2  relative  to  Ia  the  better  is  the  speed  regula- 
tion. This  method  of  control  is  used  only  where  slow  speeds  are 
required  for  short  intervals. 


CHAPTER  XIX 


HAND-OPERATED  FACE  PLATE  STARTERS  AND    CON- 
TROLLERS 

If  a  switch  is  opened  in  a  circuit  carrying  current,  an  arc  will 
be  formed  and,  unless  proper  precautions  are  taken,  the  switch 
contacts  will  be  burned. 

135.  Knife  switches  such  as  that  shown  in  Fig.  143  are 
seldom  used  to  open  a  circuit  through  which  current  is  flowing; 
they  are  used  to  isolate  a  circuit  after  the  current  has  been  reduced 
to  zero. 

The  quick-break  switch  shown  in  Fig.  134  has  a  main  contact 


FIG.  134. — Quick  break  type  of  knife  switch. 

blade  A  and  an  auxiliary  contact  blade  B  held  together  by 
the  spring  C.  When  this  switch  is  opened,  the  blade  B  is  re- 
tained by  friction  until  A  has  been  withdrawn,  the  spring  C 
then  pulls  out  the  blade  B  so  quickly  that  no  appreciable  arc  is 
formed.  This  quick -break  principle  in  different  forms  is  largely 
used  when  circuits  carrying  current  have  to  be  opened. 

136.  Auxiliary  Carbon  Contacts. — In  the  switch  shown  in  Fig. 
135,  the  main  contact  blocks  a  and  b  are  bridged  by  an  arch  c  of 
leaf  copper,  and  an  auxiliary  carbon  contact  d  is  in  parallel  with 
the  contact  a.  When  this  switch  is  opened,  the  contact  a  is  the 
first  to  be  broken,  but  the  circuit  is  not  interrupted  since  current 
can  still  pass  through  the  contact  d.  This  latter  contact  is  broken 

114 


ART.  137]         HAND-OPERATED  FACE  PLATE  STARTERS 


115 


when  the  switch  opens  further,  and  an  arc  is  formed  which  burns 
the  carbon  tips.  Since  these  tips  volatilize  without  melting, 
they  remain  in  fairly  good  shape  and,  when  badly  burned,  can 
readily  be  replaced;  the  carbon  contacts  have  the  additional 


FIG.  135. — Circuit  breaker  with  auxiliary  carbon  contacts. 

advantage  that  by  their  means  a  comparatively  high  resistance 
is  inserted  in  the  circuit  and  the  current  is  reduced  before  the 
circuit  is  broken. 

137.  Blow-out  Coils. — If  the  conductor  db,  Fig.  136,  is  carrying 


FIG.  136. — Principle  of  the  blow-out  coil. 

current  and  is  in  the  magnetic  field  NS,  it  is  acted  on  by  a  force 
which,  according  to  the  left-hand  rule,  page  7,  tends  to  move  it 
upward.  If  db  is  an  arc  formed  between  two  contacts  x  and  y 
as  they  are  separated,  this  arc  will  be  forced  upward  and  will 


116         PRINCIPLES  OF  ELECTRICAL  ENGINEERING    [CHAP.XIX 


lengthen  and  break.     The  coil  A  which  produces  the  magnetic 
field  is  called  the  magnetic  blow-out  coil. 

The  application  of  this  principle  to  a  contactor  switch  is  shown 
in  Fig.  137.  The  blow-out  coil  A  produces  the  magnetic  field 
NS,  so  that  if  an  arc  passes  between  the  switch  contacts  it  will  be 
forced  upward  and  broken  on  the  contact  tips.  The  polarity  of 
the  magnetic  field  must  be  such  that  the  arc  is  blown  away  from 
the  switch  contacts  and  not  into  them. 


A 


A  Complete  switch  B  Dismantled  switch 

FIG.  137. — Contactor  switch. 

138.  Horn  Gaps. — The  intense  heat  of  an  arc  causes  convection 
currents  of  air  to  flow  upward  so  that,  if  the  switch  jaws  are 
shaped  as  shown  in  Fig.  138,  the  arc  stream  will  be  blown  upward 
and  will  finally  break  between  the  arcing  tips  c,  which  tips  may  be 
removable. 

This  effect  may  be  exaggerated  by  enclosing  the  contact  in  an 


ART.  142]         HAND-OPERATED  FACE  PLATE  STARTERS         117 


arc  chute  of  such  shape  that  the  gases,  expanding  suddenly,  can 
pass  out  only  through  the  switch  contacts.  Such  arc  chutes,  as 
for  example  that  in  diagram  A,  Fig.  137,  when  combined  with 
magnetic  blgw-out  coils,  are  very  effective. 

139.  Fuses  are  used  to  protect  electric  circuits  from  overloads. 
A  fuse  is  a  piece  of  metal  of  such  size  and  composition  that  it 
will  melt  and  open  the  circuit  when  the  current  flowing  becomes 
large  enough  to  endanger  the  circuit. 

The  melting  of  a  fuse  is  accompanied  by 
an  arc  and  by  spattering  of  the  fused  metal 
so  that  it  is  generally  advisable  to  mount 
the  fuse  in  the  center  of  a  fiber  tube  and 
surround  it  with  a  fireproof  powder  to 
quench  the  arc,  terminals  being  supplied  as 
at  a,  Fig.  143,  so  that  the  fuse  may  readily 
be  removed  and  replaced.  A  10  amp.  fuse 
is  expected  to  carry  12.5  amp.  continuously 
and  20  amp.  for  a  period  not  greater  than 
2  minutes. 

140.  Circuit    Breakers. — Circuits    which 
are  subject  to  frequent  overloads  are  gen- 
erally    protected     by     automatic     circuit 
breakers  such   as  that  shown  in  Fig.  135, 

rather  than  by  fuses.  The  operation  of  such  a  circuit  breaker  has 
been  described  on  page  40. 

141.  Motor  Starters.— The  requirements  of  a  motor  starter 
have  been  discussed  on  pages  85  and  87.     These  requirements 
have  been  met  in  many  different  ways  but  it  is  impossible  in  the 
space  available  to  describe  more  than  a  few  standard  types. 

142.  The  sliding  contact  type  of  starter,  as  used  for  shunt 
motors,  is  shown  in  Fig.  109.     As  the  contact  arm  A  is  moved  from 
the  starting  position  Ai  to  the  running  position  Az  the  following 
operations  are  performed: 

1.  The  field    coils    are  fully    excited    as    soon  as    contact   is 
made  with  the  first  segment. 

2.  The  starting  resistance  is  gradually  cut  out  as  the  contact 
arm  is  moved  from  Aito  A2;  during  this  interval  of  time  the  motor 
should  come  up  to  speed. 

3.  The  contact  arm  is  held  in  the  running  position  by  the  no- 
voltage  release  magnet  M. 

To  stop  the  motor,  the   main  switch  is   opened.     This   de- 


\ 


FIG.  138. — Remova- 
ble horn  tips  for  con- 
tactor switches. 


118      PRINCIPLES  OF  ELECTRICAL  ENGINEERING      [CHAP,  xix 

energises  the  release  coil  M;  the  contact  arm  is  then  pulled 
back  to  the  starting  position  by  the  spring  S. 

143.  Starting  Resistance. — The  resistance  used  with  a  starter 
must  have  sufficient  current  carrying  capacity  to  allow  the  motor 
with  which  it  is  used  to  start  up  every  4  min.  for  an  hour  with- 
out overheating,  the  load  being  such  that  the  motor  shall  come 
up  to  speed  in  less  than  15  sec.  with  a  starting  current  not  greater 
than  1.5  times  full-load  current. 

144.  Overload  Release. — The  current  in  a  shunt  motor  in- 
creases with  the  load  and  may  damage  the  machine  if  the  load 


FIG.  139.  FIG.  140. 

Starting  boxes  with  both  a  no-voltage  and  an  overload  release. 

becomes  too  large.  To  protect  the  motor,  an  overload  release 
should  be  supplied  to  cut  the  machine  out  of  operation  as  soon 
as  the  load  becomes  excessive.  This  overload  release  may  take 
the  form  of  fuses  or  of  a  circuit  breaker,  placed  in  the  circuit  as 
shown  in  Fig.  143,  or  it  may  consist  of  a  special  attachment 
on  the  starter  as  shown  in  Fig.  139. 

In  this  type  of  starter  the  contact  arm  A  performs  the  functions 
of  a  starting  arm  while  the  arm  B,  used  to  close  the  main  circuit 
at  C  is  connected  to  A  by  the  spring  S.  To  operate  this  starter, 


ART.  145]        HAND-OPERATED  FACE  PLATE  STARTERS 


119 


the  contact  C  is  closed  by  the  arm  B  and  is  held  closed  by  the 
latch  L,  the  arm  A  is  then  moved  over  the  contact  buttons  so  as 
to  cut  out  the  armature  resistance,  until  it  makes  contact  with 
the  no-voltage  release  magnet  M  by  which  it  is  held  againt  the 
tension  of  the  spring  S. 

The  latch  L  is  released  by  the  plunger  p  which  is  lifted  when 
the  line  current  passing  round  the  solenoid  0  reaches  a  prede- 
termined value;  the  arm  B  then  flies  up,  opens  the  main  circuit 
at  C  and  at  the  same  time  deenergizes  the  magnet  M .  Before 
the  motor  can  be  started  again  the  circuit  must  be  closed  at  C; 
the  spring  S  at  the  same  time  returns  the  contact  arm  A  to  the 
starting  position  thereby  inserting  the  starting  resistance  in  the 
armature  circuit. 

Another  type  of  overload  release  is  shown  in  Fig.  140.  In 
this  type,  the  motor  current  passes  round  the  magnet  0  and, 


ARCING  CONTACTS 
MAIN  CONTACTS 


C      d 


A  Complete  starter  with  resistance       B  Diagrammatic  representation 
FIG.  141.  —  Multiple  switch  type  of  starter. 

when  it  reaches  a  predetermined  value,  the  arm  p  is  lifted  to 
close  the  contacts  tt,  the  no-voltage  release  coil  M  is  thereby 
short  circuited,  the  exciting  current  no  longer  passes  around  it, 
and  a  spiral  spring  in  the  hub  brings  the  contact  arm  A  back 
to  the  starting  position  and  cuts  the  motor  out  of  circuit.  Such  an 
overload  release  is  shown  on  the  starter  in  Fig.  143  and  may  be 
adjusted  to  open  the  circuit  for  any  current  up  to  1.5  times  full- 
load  current.  This  type  of  release  is  cheaper  than  that  in  Fig.  139 
but  it  has  the  disadvantage  that  it  does  not  protect  the  motor 
during  the  starting  period. 

145.  Multiple  Switch  Starters.  —  The  sliding  contact  type  of 
starter  is  liable  to  give  trouble  due  to  arcing  at  the  contacts  if 
used  for  motors  larger  than  35  h.p.  at  110  volts  or  50  h.p.  at 


120        PRINCIPLES  OF  ELECTRICAL  ENGINEERING    [CHAP.XIX 

220  to  500  volts;  for  such  service  the'  multiple  switch  type 
shown  in  Fig.  141  is  to  be  preferred. 

With  this  type  of  starter,  each  step  of  the  armature  resistance 
is  cut  out  by  a  separate  lever  and  the  levers  are  so  interlocked 
that  they  cannot  be  closed  except  in  the  proper  order,  thus  switch 
6  cannot  be  closed  before  a  because  of  the  stop  s.  Starting  with 
a,  the  switches  are  closed  hand  over  hand,  each  switch  as  it  is 
closed  keeping  the  'one  to  the  left  of  it  in  contact  by  means  of 
the  stops  s;  the  last  switch  e  is  held  closed  by  the  latch  /.  Such 
a  starter  cannot  be  left  partly  closed  with  part  of  the  resistance  in 
circuit  because  none  of  the  switches  can  stay  closed  until  the  last 
switch  e  is  latched. 

When  the  circuit  is  interrupted,  the  magnet  M  is  deenergized, 
the  latch  /  is  released,  and  the  switch  e  opens;  the  other  switches 
then  open  one  after  the  other. 

146.  Compound  Starters. — The  speed  of  a  shunt  motor  may 
be  adjusted  by  means  of  a  rheostat  in  the  field  circuit  (page 
89).  When  this  rheostat  is  incorporated  in  the  starter,  as 
shown  in  Fig.  142,  the  resulting  piece  of  apparatus  is  called  a 
compound  starter. 

The  field  contact  arm  A  and  the  armature  contact  arm  B  are 
mounted  on  the  same  hub  post;  there  is  a  spiral  spring  in  the  hub 
of  B  but  none  in  A.  To  start  the  motor  the  two  arms,  which  are 
interlocked,  are  moved  over  together,  their  motion  being  opposed 
by  the  spiral  spring,  and  the  armature  resistance  is  gradually  cut 
out  by  the  arm  B  which  finally  makes  contact  with  the  no-voltage 
release  magnet  M  by  which  it  is  held.  The  contact  arm  A  is  then 
free  to  move  backward  over  the  field  contacts  thereby  weakening 
the  shunt  field  and  increasing  the  speed  of  the  motor.  During 
the  operation  of  starting,  the  field  resistance  is  short  circuited  by 
the  switch  s  which  is  kept  closed  by  a  spiral  spring  in  the  hub  h; 
this  short  circuit  is  removed  while  the  starting  arm  B  moves  on  to 
the  last  contact  c,  the  switch  s  being  then  tripped  by  the  projec- 
tions g. 

When  the  circuit  is  interrupted,  the  magnet  M  is  deenergized, 
and  the  arm  B  is  pulled  back  by  the  spiral  spring  in  the  hub 
and  carries  the  field  arm  with  it.  With  such  a  starter  it  is  im- 
possible to  start  up  except  with  full  field,  it  is  also  impossible  to 
leave  the  starting  arm  B  in  any  position  intermediate  between 
the  starting  and  the  running  positions. 

When  a  motor  has  to  be  operated  with  a  weak  magnetic  field, 


ART.  147]         HAND-OPERATED  FACE  PLATE  STARTERS         121 


for  adjustable  speed  operation,  it  is  generally  advisable  to  con- 
nect the  no-voltage  release  coil  M  directly  across  the  line  so  that 
its  holding  power  is  not  weakened  when  resistance  is  put  in  the 
field  coil  circuit. 


FIG.  142. — Compound  starter. 

147.  Speed  Regulators. — To  obtain  speeds  lower  than  normal 
a  resistance  must  be  inserted  in  series  with  the  armature,  see  page 
89.  Any  of  the  starters  already  described  can  be  used  as  a 
speed  regulator  if  the  resistance  is  able  to  carry  the  current  of  the 
machine  continuously  without  overheating  and  if  provision  is 
made  to  return  the  contact  arm  to  the  starting  position  should  the 
voltage  fail. 

A  sliding  contact  type  of  speed  regulator  is  shown  in  Fig.  143. 
Resistance  in  the  armature  circuit  is  used  to  cut  down  the 
speed  below  normal,  while  speeds  higher  than  normal  are  ob- 
tained by  inserting  Resistance  in  the  field  coil  circuit  on  the  steps 
between  c  and  d. 

This  regulator  operates  in  exactly  the  same  way  as  a  starter  of 
the  sliding  contact  type  except  that  the  handle  can  stay  on  any 
contact  and  can  still  be  released  from  that  contact  by  the  no- 


122      PRINCIPLES  OF  ELECTRICAL  ENGINEERING      [CHAP,  xix 


voltage  release.  Attached  to  the  starting  arm  is  the  circular 
ratchet  C  which  moves  with  the  arm  and,  engaging  in  the  ratchet 
from  below,  is  a  pawl  which  is  caused  to  press  against  the  ratchet 
by  the  no-voltage  release  magnet  M  and  to  notch  into  the  ratchet 
when  the  contact  arm  is  on  a  contact  segment.  Should  the  circuit 
be  interrupted,  the  magnet  M  is  deenergized,  the  pawl  is  released, 
and  the  starting  arm  is  returned  to  the  off  position  by  a  spring 
in  the  hub. 


FIG.  143. — Sliding  contact  type  of  speed  regulator. 

148.  Controllers  for  Series  Motors. — Such  motors  are  largely 
used  for  crane  service  and  the  controller  used  with  them  must  be 
arranged  to  reverse  the  direction  of  rotation  by  reversing  the 
armature  connections  and  must  also  give  speed  regulation  by 
means  of  resistance  in  the  armature  circuit. 

A  simple  type  of  controller  for  this  purpose  is  shown  in  Fig.  144. 
The  path  of  the  current  through  the  controller  is  shown  in 
diagram  A  when  the  motor  is  running  in  one  direction  and  in 
diagram  B  when  the  direction  is  reversed.  , 

The  controller  shown  in  Fig.  144  is  supplied  with  a  blow-out 
coil  A  which  produces  a  magnetic  field  that  passes  from  the  front 
arms  B  to  the  back  arms  C,  which  arms  are  of  iron.  This  magnetic 
field  passes  vertically  through  the  slate  front  and  so  is  at  right 


ART.  148]         HAND-OPERATED  FACE  PLATE  STARTERS         123 


angles  to  the  arc  formed  when  a  sliding  contact  leaves  a  segment, 
it  therefore  acts  to  blow  out  the  arc. 


/   \ 


A  Hoisting  B  Lowering 

FIG.  144. — Face  plate  controller  for  a  small  reversing  series  motor. 

The  operation   of   crane   and    hoist   motors   is   taken   up   in 
greater  detail  in  Chapter  XL. 


CHAPTER  XX 


DRUM  TYPE  CONTROLLERS 

149.  Drum  type  controllers  are  particularly  suited  for  adjustable 
speed  motors  which  have  to  be  started  and  stopped  frequently, 
because  the  various  operations  .are  performed  readily  by  the  move- 
ment of  a  single  handle  and  take  place  in  their  proper  order  The 
controller  is  entirely  enclosed  and  can  readily  be  made  weather- 
proof, while  contact  with  the  live  parts  is  prevented. 

A  simple  type  of  drum  controller  is  shown  in  Fig.  146.  It 
consists  of  a  cast-iron  drum  cylinder  A ,  insulated  from  a  central 
shaft  to  which  the  operating  handle  B  is  keyed.  To  this  drum, 


FIG.  145. — Developed  diagram  of  machine      FIG.  146. — Machine  tool  con- 
tool  controller.  troller. 

the  copper  contact  segments  a,  6,  c,  d  and  e  are  attached ;  these 
are  in  electrical  contact  with  the  drum  and  therefore  with  one 
another.  .  The  drum  carries  also  a  brush  contact  m  which  slides 
over  stationary  field  resistance  contacts  that  are  mounted  on 
the  slate  C;the  contact  m  is  not  visible,  being  hidden  by  the  drum. 
The  armature  resistance  is  connected  to  the  stationary  fingers 
f>  9)  h,  j  and  k  which  are  insulated  from  one  another  and  mounted 
on  a  wooden  base. 

The  action  of  such  a  controller  may  readily  be  understood 
from  Fig.  145  which  shows  the  controller  drum  developed  on  to 

124 


ART.  150] 


DRUM  TYPE  CONTROLLERS 


125 


a  plane;  the  vertical  dotted  lines  indicate  the  successive  positions 
of  the  contact  drum  with  respect  to  the  row  of  stationary 
fingers. 

In  position  1,  the  fingers/  and  g  make  contact  with  segments 
a  and  b  of  the  drum,  and  the  armature  current  passes  through 
the  whole  armature  resistance,  while  the  field  coils  are  fully  excited, 
the  exciting  current  passing  through  the  contact  m.  In  position 
4,  the  armature  resistance  is  all  cut  out  but  the  field  coils  are  still 
fully  excited.  In  position  5,  the  brush  m  makes  contact  with  field 
segment  5  and  the  resistance  n  is  inserted  in  the  field  coil  circuit. 
With  further  motion  of  the  drum  from  position  5  to  position  13, 
the  resistance  in  the  field  coil  circuit  is  gradually  increased,  the 
magnetic  field  is  weakened,  and  the  speed  of  the  motor  is  thereby 
increased  above  normal. 

150.  No-voltage  and  Overload  Release.— The  controller 
shown  in  Fig.  146  is  not  provided  with  either  a  no-voltage  or  an 


Tr 


FIG.    147. — No-voltage   and      FIG.    148. — Connections  of  no-voltage  and 
overload  release  panel.  overload  release  panel. 

overload  release.  These  are  sometimes  incorporated  in  the 
controller  but  are  more  often  supplied  separately  on  a  panel  such 
as  that  shown  in  Fig.  147,  the  equipment  consisting  of  a  single 
pole  magnetic  switch  A  and  an  overload  release  coil  B.  The 
connections  of  this  panel  are  shown  diagrammatically  in 
Fig.  148. 

The  contact  b  is  kept  open  by  means  of  a  spring.  When  this 
contact  is  closed,  current  passes  through  the  control  circuit 
abcdefgh,  from  the  positive  to  the  negative  side  of  the  line,  and 


126      PRINCIPLES  OF  ELECTRICAL  ENGINEERING       [CHAP,  xx 


excites  the  electromagnet  M,  which  closes  the  switch  A  and 
allows  current  to  pass  to  the  motor.  When  the  switch  A  closes, 
the  contact  s  also  closes  and  current  now  passes  through  the 
circuit  ascdefgh  and  excites  the  magnet  M,  so  that  the  contact  6 
can  be  opened.  In  this  latter  circuit  is  a  resistance  r,  which 
reduces  the  current  in  the  holding  coil  M  after  the  switch  A  has 
been  closed  so  that  this  switch  can  open  the  more  readily  should 
power  go  off  from  the  line;  this  magnetic  switch  A  therefore  acts 
as  a  no-voltage  release. 

The  total  current  in  the  machine  passes  round  the  overload 
coil  B  and,  when  this  current  reaches  a  predetermined  value, 
the  plunger  p  is  raised  to  strike  the  lever  q  and  open  the  control 
circuit  at  e,  thereby  deenergizing  the  magnet  M  and  allowing 
the  switch  A  to  open. 


27 


A    Motors  in  Parallel  B    Motors  in  Series 

FIG.  149. — Series-parallel  system  of  motor  control. 

To  stop  the  motor,  the  contact  d  is  opened;  this  opens  the 
control  circuit  and  allows  the  main  switch  A  to  open. 

The  contacts  6  and  d  can  be  embodied  in  the  controller  in 
such  a  way  that  the  contact  b  and  therefore  the  switch  A  cannot 
be  closed  except  when  the  controller  handle  is  in  the  off  position 
and  all  the  armature  resistance  is  inserted  in  the  armature 
circuit. 

151.  Street  Car  Controller  for  Series  Parallel  Control.— 
Street  cars  are  equipped  with  series  motors,  and  the  number  of 
motors  per  car  is  a  multiple  of  two.  To  start  these  machines,  a 
resistance  is  placed  in  series  with  the  armatures  as  shown  in 
diagram  A,  Fig.  149,  and  is  then  gradually  cut  out  as  the  motors 
come  up  to  speed. 

The  torque  required  to  start  and  accelerate  a  car  is  much 
greater  than  that  required  to  keep  the  car  in  motion  so  that  the 
current  /  in  the  motor  is  large  at  starting  and,  if  the  motors  are 


ART.  151| 


DRUM  TYPE  CONTROLLERS 


127 


connected  as  shown  in  diagram  A,  a  large  current  21  is  taken 
from  the  line.  To  reduce  this  current  for  half  of  the  starting 
period,  the  series  parallel  method  of  control  is  adopted. 


FIG.  150. — Street  railway  controller. 

During  the  first  half  of  the  starting  period,  the  motors  are  con- 
nected in  series,  as  shown  in  diagram  B,  so  that  the  total  line 
current  passes  through  both  machines;  when  the  starting  re- 
sistance is  all  cut  out,  each  motor  has  half  of  the  normal  voltage 


8    9    10  11  12 


Series  connectioji          Complete  diagram  of  connections       Parallel  connec- 
tion 
FIG.  151. — Diagram  of  connections  of  a  street  railway  controller. 

applied  across  its  terminals  and  runs  at  half  speed.  To  obtain 
higher  speeds,  the  motors  are  now  connected  in  parallel  and  the 
resistance  is  again  inserted  in  the  circuit  as  shown  in  diagram  A; 


128       PRINCIPLES  OF  ELECTRICAL  ENGINEERING      [CHAP,  xx 


this  resistance  is  gradually  cut  out  as  the  motors  come  up  to  speed 

and,  when  it  is  all  cut  out,  each  motor  is  running  on  normal  line 

voltage. 

These  operations  are  performed  with  a  drum  controller  of  the 

type  shown  in  Fig.  150,  which  controller  is  shown  developed  in 

Fig.  151. 

x  Y  The  direction  of  the   cur- 

; rent  when  the  controller  is  in 

position  1  is  shown  in  dia- 
gram A.  The  current  passes 
through  the  whole  armature 
resistance  R  and  then  through 
the  motors  in  series.  The 
armature  resistance  is  gradu- 
ally cut  out  as  the  controller 


9    D-J  p'TOir* — 


FIG.  152. — Diagram  of  connections  of 
reversing  drum. 


is  moved  to  position  5,  when 
the  motors  are  in  series  across 
the  line  and  are  running  at  half  speed. 

The  direction  of  the  current  when  the  controller  is  in  position  8 
is  shown  in  diagram  B.  The  current  passes  through  the  whole 
armature  resistance  and  then  divides  up  and  passes  through  the 


FIG.  153. — Section  through  drum  type  of  controller,  showing  the  blow  out 

coil. 

two  motors  in  parallel.  The  resistance  is  gradually  cut  out  as  the 
controller  is  moved  to  position  12,  the  motors  are  then  in  parallel 
across  the  line  and  are  running  at  full  speed. 

The  complete  controller  is  shown  developed  in  diagram  C. 
The  student  should  draw  the  drum  on  tracing  paper  and  move  it 
over  the  stationary  contacts  and  note  how  the  various  combina- 
tions are  obtained. 


ART.  153]  DRUM  TYPE  CONTROLLERS  129 

152.  Reversing  Drum. — To  reverse  the  motors,  the  armature 
connections  are  reversed  relative  to  the  field  connections.     This  is 
done  by  means  of  the  auxiliary  drum  B,  Fig.  150,  which  is  separate 
from  the  main  drum  but  is  so  interlocked  with  it  that  the  motors 
cannot  be  reversed  except  when  the  main  drum  is  in  the  off  posi- 
tion.    This  drum  is  shown  developed  in  Fig.  152. 

When  the  reversing  drum  is  in  position  X,  current  passes 
through  the  armature  from  a  to  6;  when  in  position  Y,  the  current 
passes  from  6  to  a.  A  double  set  of  such  contacts  are  required 
for  a  pair  of  motors. 

153.  Mechanical  Features   of  Drum   Controllers. — The  con- 
troller frame  is  of  cast  iron  and  has  an  asbestos  lined  removable 
cover  A,  Fig.  150;  the  back  of  the  frame  is  pierced  with  a  verti- 
cal row  of  holes  lined  with  insulating  bushings  through  which 
the  necessary  leads  run. 

The  contact  .cylinder  D  with  the  copper  contacts  is  supported 
by  and  insulated  from  a  central  shaft  to  which  the  operating 
handle  is  attached. 

The  blow-out  coil  is  shown  at  C.  Hinged  to  the  case  of  the  blow- 
out magnet  is  a  steel  plate  E  extending  vertically  the  entire 
length  of  the  cylinder  and  constituting  such  a  magnetic  circuit 
that  a  powerful  magnetic  field  <f>  is  maintained  across  the  finger 
contacts  as  shown  in  Fig.  153.  This  steel  plate  is  lined  with  as- 
bestos on  its  inner  side  and  carries  moulded  refractory  insulating 
arc  barriers  F  which  project  between  the  contact  rings. 


CHAPTER  XXI 


AUTOMATIC  STARTERS  AND  CONTROLLERS 

The  tendency  in  the  operation  of  electrical  machinery  is  to 
make  the  starters  and  controllers  self  governing  so  that  the 
machinery  cannot  be  injured  by  careless  or  unskilled  operators. 

154.  Automatic  Solenoid  Starter. — Fig.   154  shows  a  sliding 
contact  type  of  starter  in  which  the  contact  arm  is  moved  by 
means  of  a  solenoid.     When  the  main  switch  K  is  closed,  the 

solenoid  A  is  excited  and  pulls 
the  contact  arm  upward 
thereby  cutting  the  resistance 
R  out  of  the  armature  circuit; 
the  rate  at  which  the  contact 
arm  moves  may  be  regulated 
by  the  dash  pot  D. 

At  the  end  of  its  travel  the 
contact  arm  presses  on  the 
stop  s  and  opens  the  contact 
c  and  thereby  inserts  the  re- 
sistance r  in  the  solenoid  cir- 
cuit so  that  the  current  in 
this  circuit  is  reduced  and  is 
not  larger  than  necessary  to 
hold  the  arm  in  the  running 
position.  When  the  power  is 
off,  or  when  the  switch  K  is 
opened,  the  solenoid  is  deen- 

ergized,  and   the  starting  arm  falls  by  gravity  to  the  starting 

position. 

155.  Float  Switch  Control. — With  such  an  automatic  starter 
it  te  not  necessary  to  run  the  power  mains  to  the  point  from  which 
the  motor  has  to  be  started.     The  main  switch  K  may  be  mag- 
netically operated  and  placed  on  the  same  panel  as  the  starter 
as  shown  in  Fig.  155.     This  switch  is  operated  by  a  control  cir- 
cuit as  shown  diagram matically  in  Fig.  156. 

130 


FIG.  154. — Automatic  solenoid  starter. 


ART.  156]       AUTOMATIC  STARTERS  AND  CONTROLLERS        131 


When  the  switch  s  is  closed,  the  magnetic  switch  K  is  excited 
and  closes  the  main  circuit  so  that  power  is  available  at  the  motor 


FIG.  155. — Automatic  solenoid  starter  with  main  magnetic  switch. 

terminals.     The  switch  s  may  readily  be  opened  and  closed  by 
means  of  a  float,  a  pressure  gauge  or  some  other  device,  so  as  to 
maintain  the  water  level   or 
the    air    pressure   in   a   tank 
within  prescribed  limits. 

When  the  float  F  in  Fig.  156 
falls  below  a  certain  point,  the 
projection  e  raises  the  lever  I 
and  W  is  moved  over.  When 
W  passes  the  vertical  position 
it  drops  over  and  the  projection 
g  snaps  the  switch  s  into  con- 
tact and  closes  the  control  cir- 
cuit. When  the  water  evel 
reaches  the  upper  limit,  the 
projection  /  trips  the  lever  W 
which  opens  the  switch  s  and  FIG.  156. — Float  switch, 

causes  the  motor  to  stop. 

156.  Magnetic  Switch  Controller. — Starters  for  large  motors 
are  of  the  multiple  switch  type,  see  page  119,  and,  when  the 


132      PRINCIPLES  OF  ELECTRICAL  ENGINEERING      [CHAP.XXI 

motors  have  to  be  operated  from  a  distance,  the  switches  are 
magnetically  operated  and  are  closed  in  their  proper  order  by  a 
master  controller  operating  on  a  control  circuit.  Fig.  157 
shows  such  a  starter  used  to  control  .a  shunt  motor  by  means  of 
resistance  in  the  armature  circuit. 

The  switches  A,  B,  C ,  D,  E  and  F  are  magnetic  contactor 
switches  which  close  when  the  electromagnets  a,  b,  c,  d,  e  and  / 
are  excited.  These  magnets  are  connected  across  the  mains 
xy  in  the  proper  order  by  means  of  a  small  drum  controller  M 
called  a  master  controller. 

As  the  drum  M  is  turned,  the  first  contacts  to  be  made  are  x 
and  1  and  the  electromagnet  /  is  excited  aiid  the  main  switch 


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FIG.  157. — Magnetic  switch  controller. 

F  is  closed.  In  the  next  position  of  the  drum,  the  contact  2  is 
closed  and  the  magnet  a  is  thereby  excited  which  closes  the 
switch  A  and  connects  one  terminal  of  the  motor  to  the  line. 
On  contact  3  being  closed,  the  magnet  b  is  excited  which  closes 
the  switch  B  and  connects  the  motor  armature  across  the  line 
with  all  the  armature  resistance  R  in  series.  With  further  motion 
.of  the  drum  M,  the  resistances  R\,  R2  and  R%  may  be  cut  out 
one  after  the  other. 

The  drum  M  has  to  handle  only  the  current  in  the  control 
circuits  which  current  is  of  the  order  of  1  amp.,  the  drum  may 
therefore  be  small  in  size. 

With  such  a  system  of  control,  practically  any  series  of  opera- 
tions can  be  performed  in  their  proper  order  and  several  motors 
can  be  controlled  from  a  single  master  controller. 


ART.  158]        AUTOMATIC  STARTERS  AND  CONTROLLERS       133 

157.  Multiple  Unit  Control  of  Railway  Motors. — The  pos- 
sibilities of  magnetic  switch  controllers  are  well  illustrated  in 
the  multiple  unit  system  of  car  control.  An  electric  train,  made 
up  of  a  number  of  cars  each  with  its  own  motors  and  magnetic 
switch  controller,  can  readily  be  controlled  by  a  single  operator 
at  the  head  of  the  train. 

The  control  circuit  carries  only  a  small  current  so  that  the  leads 
are  light  and  flexible;  these  leads  are  carried  the  whole  length  of 
the  train.  As  each  contact  of  the  master  controller  is  closed, 
the  corresponding  electromagnets  under  each  car  are  excited 
and  the  switches  closed.  If  for  example,  in  Fig.  158,  the  contacts 


-,  

Ma 

Contr 

W,            1 

I/.       1 

w/.         1 

ffl/ 

FIG.  158. — Principle  of  the  multiple  unit  system  of  control. 

a  and  b  of  the  control  circuit  are  energized  by  the  master  controller 
then  the  magnetic  switches  A  on  each  of  four  separate  cars  will 
close  simultaneously. 

158.  Automatic  Magnetic  Switch  Starters. — With  certain 
automatic  features  attached,  the  magnetic  switch  starter  may  be 
so  constructed  that  the  operator  has  only  to  close  the  main  switch, 
after  which  the  starting  resistance  is  cut  out  automatically  by  the 
controller  and  the  motor  is  brought  up  to  speed  without  the 
starting  current  exceeding  a  predetermined  value. 

The  operation  of  such  automatic  starters  depends  on  the 
current  changes  in  the  armature  circuit  when  the  motor  with 
which  the  starter  is  used  is  brought  up  to  speed.  In  the  hand 
operated  starter  in  Fig.  159,  when  the  switch  A  is  closed,  a  current 
of  about  one  and  a  half  times  full-load  current  flows  through  the 
armature  and  the  starting  resistance  in  series,  and  the  motor 
starts  up.  As  it  gains  in  speed,  the  back  e.m.f.  increases  and  the 
current  drops  and,  when  this  current  has  reached  full-load 
value,  the  switch  B  is  closed  and  the  same  current  cycle  is 


134      PRINCIPLES  OF  ELECTRICAL  ENGINEERING     [CHAP,  xxi 

again  passed  through.     Fig.  160  shows  the  current  cycle  for  a 
four  switch  starter  such  as  that  in  Fig.  159. 

In  the  automatic  starter  shown  diagrammatically  in  Fig.  161, 
when  the  switch  p  is  closed,  the  solenoid  a  is  excited  and  the 
contractor  switch  A  is  closed  thereby  connecting  one  terminal  of 
the  motor  to  the  positive  side  of  the  line  and  at  the  same  time 


— 4      y — fw^v^^wwvwiA/vi 

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FIG.  159.— Multiple       FIG.  160. — Current  cycle  in  the 
switch  type  of  starter,    armature  circuit  during  starting. 

closing  the  field  coil  circuit  and  exciting  the  machine;  the  field  coil 
circuit  is  not  shown  in  Fig.  161. 

The  switch  A,  when  open,  supports  the  relay  e  and  when  A 
closes  e  is  dropped  and  closes  the  contacts  /  so  that  the  solenoid  b 
is  excited  and  the  switch  B  is  closed  thereby  connecting  the  other 


FIG.  161. — Automatic  starter  with  shunt  magnetic  switches  and  series  relays. 

terminal  of  the  motor  to  the  negative  side  of  the  line  with  all  the 
starting  resistance  in  series.  Current  then  passes  through  the 
armature,  Rs,  R2,  Ri,  m  and  q. 

When  the  switch  B  closes,  a  current  of  about  one  and  a  half 
times  full-load  current  flows  through  the  armature,  and  the  motor 
starts  up.  Now  the  solenoid  g  is  connected  across  the  resistance 


ART.  159]        AUTOMATIC  STARTERS  AND  CONTROLLERS       135 

R,  so  that  the  voltage  across  this  coil  is  the  drop  across  the  resist- 
ance, and  the  current  which  it  sends  through  the  coil  is  large 
enough  to  hold  up  the  plunger  g  even  although  the  mechanical 
support  was  removed  when  the  switch  B  closed.  As  the  motor 
gains  in  speed,  its  back  e.m.f.  increases  and  the  drop  across  the 
resistance  decreases  so  that  the  current  in  coil  g  decreases  and, 
when  this  current  has  reached  a  predetermined  value,  the 
plunger  of  g  drops  and  closes  the  contacts  h,  the  solenoid  c  is 
thereby  excited  and  the  switch  C  is  closed  cutting  out  Ri,  the  first 
step  of  the  resistance. 

After  B  closed  but  before  c  was  excited,  the  motor  current 
passed  through  the  solenoid  m  and  held  up  this  plunger,  but  after 
the  switch  C  has  closed,  this  current  passes  through  only  the  lower 
half  of  coil  m.  At  the  instant  C  closes,  the  current  is  large  and 
although  passing  through  only  half  of  coil  m  it  still  holds  the  relay 
open.  As  the  motor  accelerates  however,  the  current  in  the 
armature  circuit  decreases  and  finally  reaches  a  value  with  which 
the  solenoid  m  is  no  longer  able  to  support  its  plunger,  which 
plunger  therefore  drops  and  closes  the  contacts  n,  the  solenoid  d 
is  thereby  excited  and  the  switch  D  is  closed  cutting  out  R2,  the 
second  step  of  the  resistance. 

The  switch  E  operates  in  exactly  the  same  way  as  D  and  cuts 
out  the  last  step  of  the  starting  resistance.  Such  a  relay  switch 
as  that  used  in  this  type  of  automatic  starter  is  shown  in  Fig.  137. 

159.  Automatic  Starter  with  Series  Switches,— The  closing 
electromagnets  of  the  starter  shown  in  Fig.  161  are  shunt  wound 
and  the  starter  is  said  to  be  of  the  shunt  switch  type  with  series 
relays. 

Another  type  of  starter  is  shown  in  Fig.  163,  the  switches  in  this 
case  carry  the  line  current  and  are  called  series  switches.  They 
are  so  constructed  that  they  will  not  close  when  the  current 
flowing  in  the  exciting  coil  exceeds  a  predetermined  value.  Such 
a  switch  is  shown  in  Fig.  162.  The  upper  end  of  the  iron  plunger 
E  carries  a  non-magnetic  stem  G  to  which  is  attached  a  copper 
plate  H  which  makes  contact  with  the  brushes  k  when  the 
plunger  is  raised. 

When  a  small  current  passes  in  the  coil  M,  the  flux  in  the 
magnetic  circuit  passes  as  shown  in  diagram  A  and  the  plunger 
tends  to  move  upward  so  as  to  reduce  the  reluctance  of  the 
magnetic  circuit. 

When  a  large  current  passes  in  the  coil,  the  flux  in  the  magnetic 


136       PRINCIPLES  OF  ELECTRICAL  ENGINEERING    [CHAP.XXI 


circuit  passes  as  shown  in  diagram  B;  the  reduced  stem  F  becomes 
highly  saturated  so  that  a  large  part  of  the  total  flux  passes  across 
the  gap  Z2  without  entering  the  stem  and  the  plunger  tends  to 
move  upward  to  reduce  the  gap  li  and  downward  to  reduce  the 
gap  12.  The  larger  the  current,  the  larger  the  value  of  02  relative 
to  0i  +  02,  and  the  less  the  tendency  for  the  plunger  to  move  up. 
With  such  a  solenoid  then,  when  the  current  exceeds  a  pre- 
determined value,  the  downward  pull  plus  the  weight  of  the 
plunger  keeps  the  plunger  from  being  lifted,  but  as  the  current  is 
decreased,  the  flux  02  and  the  downward  pull  both  decrease 


A  B 

FIG.  162. — Series  automatic  switches. 

rapidly  as  the  stem  F  ceases  to  be  saturated,  until  finally  the 
upward  pull  is  able  to  raise  the  plunger. 

The  critical  value  of  current  with  which  the  plunger  can  be 
lifted  may  be  adjusted  by  raising  or  lowering  the  iron  plug  C 
so  as  to  change  the  value  of  02  relative  to  that  of  0i  +  02- 

A  starter  made  with  three  such  switches  is  shown  in  Fig.  163. 
When  the  switch  A  is  closed,  a  large  current  flows  through  the 
armature,  the  starting  resistance  and  the  coil  C\  in  series  and  the 
motor  starts  up;  the  switch  #1  however  does  not  close  until  the 
motor  has  gained  in  speed  and  the  current  has  dropped  to  the 
value  for  which  the  solenoid  Ci  was  set. 

When  Ci  closes,  the  first  step  of  the  starting  resistance  is  cut 
out  and  the  current,  which  increased  considerably  when  the 
switch  closed,  now  passes  through  C\  and  Cz  and  the  two  re- 
maining steps  of  the  resistance.  This  current  decreases  as  the 


ART.  159]       AUTOMATIC  STARTERS  AND  CONTROLLERS        137 

motor  speeds  up  and  then  C2  closes    the    contacts  #2.     The 
same  current  cycle  is  again  passed  through  after  which  C3  closes 


FIG.  163. — Automatic  starter  with  series  switches. 

and  cuts  out  the  last  step  of  the  starting  resistance.     The  con- 
tact #3  is  kept  closed  by  means  of  the  small  shunt   solenoid  s. 


FIG.  164. — Automatic  starter  with  three  switches  and  with  the  starting 

resistance. 

The  contacts  g3  a're  the  only  ones  that  carry  current  continu- 
ously because,  when  C3  closes,  current  no  longer  passes  through 


138      PRINCIPLES  OF  ELECTRICAL  ENGINEERING     [CHAP,  xxi 

Ci  and  C2  because  it  has  an  easier  path  through  the  circuit  abc, 
the  plungers  of  Ci  and  C2  therefore  drop. 

When  the  main  switch  A  is  opened,  the  current  in  the  motor 
circuit  drops,  but  the  switch  C3  requires  only  a  small  current  to 
hold  up  the  plunger  which  therefore  does  not  drop  until  the 
current  has  become  almost  zero,  so  that  blow-out  coils  are  not 
required  to  protect  the  contacts  gs.  A  complete  starter  is 
shown  in  Fig.  164. 

The  no-voltage  and  overload  release  attachments  are  generally 
supplied  on  a  separate  panel  such  as  that  shown  in  Fig.  147, 
page  125,  the  main  switch  A  being  then  of  the  magnetic  switch 
type  is  operated  from  a  push-button  circuit. 


CHAPTER  XXII 
ELECTROLYSIS  AND  BATTERIES 

160.  Electrolysis. — Certain  liquids  conduct  electricity  but  in 
doing  so  they  undergo  decomposition.     Such  liquids  are  called 
electrolytes  and  include  bases,  acids  and  salts,  in  solution  or  in 
the  molten  state.     The  conductors  by  which  the  current  enters 
or  leaves  the  electrolyte  are  called  electrodes;  that  connected  to 
the  positive  line  terminal  is  called  the  anode  and  is  the  one  at 
which  the  current  enters,  the  other  is  called  the  kathode.     The 
name  electrolysis  is  given  to  the  whole  process. 

It  would  seem  that  the  electrolyte,  in  addition  to  containing 
complete  molecules  of  the  substance  in  solution,  contains  also 
molecules  which  are  dissociated  into  ions  (atoms  carrying  positive 
or  negative  charges)  and  that  the  metal  and  hydrogen  atoms 
carry  positive  charges  while  non  metals,  the  hydroxyl  group 
(OH)  and  the  acid  radicals  (SO  4,  N03,  etc.)  carry  negative 
charges.  If  then  a  difference  of  potential  is  established  between 
the  electrodes,  the  positively  charged  ions  will  be  attracted  to 
the  negative  electrode  and  the  negatively  charged-ions  to  the 
positive  electrode,  where  they  give  up  their  charges.  When 
this  occurs,  the  particle  or  group  ceases  to  be  an  ion  and  dis- 
plays at  once  its  ordinary  chemical  properties. 

If  a  direct  current  is  passed  through  a  solution  of  hydrochloric  acid  (HC1), 
using  platinum  electrodes,  then  the  +  H  ions  will  be  attracted  to  the  nega- 
tive electrode  where  they  will  give  up  their  charges  and  then  appear  as  hydro- 
gen gas,  similarly  the  —  Cl  will  appear  at  the  positive  electrode. 

If  a  solution  of  sulphuric  acid  (H2SO4)  is  used,  then  the  -fH  will  be  liber- 
ated at  the  negative  electrode  and  the  —  SO 4  at  the  positive  electrode. 
The  SO 4,  however,  acts  on  the  water  of  the  solution  to  form  sulphuric  acid 
and  oxygen,  which  latter  gas  is  liberated  while  the  acid  goes  into  the  solution. 
If  the  positive  electrode  had  been  of  copper,  then  the  SO4  would  have  acted 
on  the  copper  to  form  copper  sulphate  which  would  have  gone  into  the 
solution. 

161.  Voltameter. — If  a  negative  electrode  of  paltinum  and  a 
positive  electrode  of  pure  silver  are  used  in  a  solution  of  silver 
nitrate  (Ag  N03),  then  silver  is  deposited  on  the  negative  plati- 

139 


140     PRINCIPLES  OF  ELECTRICAL  ENGINEERING     [CHAP.XXII 

num  electrode  and  the  —  NO3  acid  radical,  liberated  at  the  posi- 
tive electrode,  acts  on  the  silver  to  form  more  silver  nitrate,  which 
goes  into  solution  and  thereby  keeps  the  concentration  of  the 
electrolyte  constant. 

Faraday's  experiments  showed  that  the  mass  of  material 
deposited  is  proportional  to  the  quantity  of  electricity  (current 
X  time)  so  that  the  apparatus  described  above,  called  the  silver 
voltameter,  can  be  used  as  a  measure  of  quantity  of  electricity. 
This  instrument  is  used  as  a  primary  standard. 

162.  Electric  Battery. — An  electric  battery  is  a  device  for 
transforming  chemical  energy  into  electrical  energy  and  consists 
essentially  of  two  dissimilar  plates  in  a  solution  which  acts  more 
readily  on  one  plate  than  on  the  other.     A  difference  of  potential 
is  found  between  two  such  plates  so  that  if  they  are  joined  by  a 
wire  an  electric  current  will  flow  in  this  wire.     The  magnitude  of 
this  e.m.f.  depends  only  on  the  material  of  the  plates  and  on  the 
electrolyte  and,  for  a  given  pair  of  plates,  is  independent  of  their 
area. 

There  are  innumerable  types  of  battery,  but  in  nearly  every 
case  one  plate  is  of  zinc  and  the  other  of  either  carbon  or  copper. 
If  a  battery  is  made  of  copper,  zinc  and  dilute  sulphuric  acid,  it 
will  be  found  that,  when  current  flows  in  a  conducting  wire  con- 
necting the  copper  and  the  zinc  plates,  the  zinc  goes  into  solution 
as  ZnSOi  while  hydrogen  is  given  off  at  the  copper  plate.  The 
zinc  and  the  sulphuric  acid  are  therefore  used  up  and  electrical 
energy  is  obtained  at  the  expense  of  the  chemical  energy  which 
was  contained  in  these  materials. 

When  the  cell  becomes  exhausted,  the  zinc  plate  and  the  elec- 
trolyte have  to  be  renewed.  When  fresh  materials  are  used,  the 
battery  is  called  a  primary  battery;  when  the  materials  are  re- 
newed by  electrolysis  in  a  way  that  shall  be  described  later,  the 
battery  is  called  a  secondary  or  storage  battery. 

163.  Theory  of  Battery  Operation. — If  we  consider  a  battery 
made  up  of  copper,  zinc  and  dilute  sulphuric  acid,  then  the  essen- 
tial difference  between  these  metals  so  far  as  battery  operation  is 
concerned  is  that  the  zinc  is  the  more  readily  acted  on  by  oxygen 
or  has  the  greater  chemical  attraction  for  oxygen  so  that,  while 
both  copper  and  zinc  attract  the  —0  ions  in  the  solution,  the 
attraction  of  the  zinc  is  the  greater.     As  both  metals  combine 
with  the  attracted  oxygen  they  become  negatively  'charged  and 
soon  repel  the  negative  oxygen  ions  as  strongly  electrically  as 


ART.  166]  ELECTROLYSIS  AND  BATTERIES  141 

they  attract  them  chemically.  When  equilibrium  is  established, 
both  metals  are  negatively  charged  but  the  negative  charge  on  the 
zinc  is  the  greater  and  its  potential  is  therefore  the  lower.  If  the 
two  metals  are  now  joined  by  a  connecting  wire  outside  of  the 
solution,  electricity  flows  from  the  copper  to  the  zinc  and  the 
plates  tend  to  come  to  the  same  potential,  so  that  the  state  of 
equilibrium  is  disturbed;  the  potential  of  the  zinc  rises  slightly 
above  its  potential  of  equilibrium  so  that  it  is  able  to  attract 
more  negative  oxygen,  while  the  potential  of  the  copper  falls 
slightly  below  its  potential  of  equilibrium  so  that  it  now  attracts 
the  positive  hydrogen  ions,  the  voltage  between  the  plates  is 
therefore  maintained  and  so  also  is  the  current  in  the  conducting 
wire. 

The  zinc  oxide  formed  is  acted  on  by  the  sulphuric  acid  to  form 
zinc  sulphate  which  goes  into  the  solution  and  is  no  longer  an 
active  constituent  of  the  cell. 

164.  Polarization. — The  action  of  this  battery  weakens  after  a 
few  minutes  of  operation,  and  this  weakening  is  found  to  be  due 
to  a  layer  of  hydrogen  bubbles  which  cling  to  the  copper  plate 
after  giving  up  their  charge.     The  cell  is  then  one  which  has  ac- 
tive plates  of  hydrogen  and  zinc  and  gives  a  much  lower  e.m.f. 
than  one  of  copper  and  zinc.     Hydrogen  has  also  a  high  electrical 
resistance  so  that  the  current  that  can  be  drawn  is  small.     This 
defect  of  the  battery  is  called  polarization,  and  different  methods 
are  used  to  keep  the  hydrogen  bubbles  away  from  the  copper 
plate,  generally  by  the  use  of  a  depolarizing  substance  containing 
an  excess  of  oxygen,  which  substance  is  placed  around  the  copper 
plate. 

165.  The  E.M.F.  and  Resistance  of  Cells. — The  electromotive 
force  depends  only  on  the  materials  of  the  cell  and  is  independent 
of  their  size,  shape  or  arrangement. 

The  internal  resistance  of  a  cell  of  given  materials  is  proportional 
to  the  distance  between  the  plates  and  inversely  proportional  to 
their  area  and,  in  order  that  a  cell  may  have  a  low  internal  resist- 
ance, the  plates  should  have  a  large  surface  and  should  be  close 
together. 

166.  The  Daniell  cell  is  a  commercial  type  of  copper,  zinc  and 
sulphuric  acid  battery  largely  used  for  telegraph  work.     The 
depolarizing  substance  in  this  cell  is  copper  sulphate  (CuSO^. 
In  one  form  of  this  battery  the  sulphuric  acid  with  the  zinc  are 
placed  in  a  porous  pot  and  this  in  turn  is  placed  in  a  saturated 


142    PRINCIPLES  OF  ELECTRICAL  ENGINEERING      [CHAP,  xxn 

solution  of  copper  sulphate  contained  in  a  glass  jar,  the  copper 
plate  is  immersed  in  this  latter  solution.  By  means  of  the  porous 
pot  the  liquids  are  kept  from  mixing  but  the  action  of  the  battery 
is  not  impaired  since  the  ions  pass  freely  through  the  walls  of  the 
pot. 

The  hydrogen  liberated  at  the  copper  electrode  acts  on  the 
adjoining  copper  sulphate  to  form  copper  and  sulphuric  acid;  the 
copper  is  deposited  on  the  electrode  so  that  there  is  no  hydrogen 
layer  formed  and  therefore  no  polarization  of  the  cell  until  all 
the  copper  sulphate  in  the  solution  has  been  exhausted. 

The  usual  size  of  Daniell  cell  has  a  voltage  on  open  circuit  of 
about  1.1  and  an  internal  resistance  of  about  2  ohms  so  the  largest 
current  that  can  be  obtained  from  such  a  cell  is  0 . 55  amperes  on 
short  circuit. 

167.  Calculation  of  the  E.M.F.  of  a  Daniell  Cell.— Faraday's 
experiments  showed  that,  when  electrolysis  takes  place,  the  num- 
ber of  gm.  of  substance  separated  out  per  coulomb  is  equal  to 

atomic  weight 

,  or  the  coulombs  required  to  produce  a  number 
96,540  X  valency 

of  grams  equal  to  the  atomic  weight  is  96,540  X  the  valency. 

Copper  (Cu)  and  zinc  (Zn)  have  a  valency  of  two  in  compounds 
such  as  CuS04  and  ZnS04,  because  one  atom  of  Cu  or  Zn  replaces 
two  atoms  of  H  from  H2SO4,  and,  since  the  atomic  weight  of  Cu 
is  63.75  and  that  of  Zn  is  65.37  therefore  96,540  X  2  coulombs 
will  separate  out  63.75  gm.  of  Cu  from  CuSO4  and  65.37  gm. 
of  Zn  from  ZnSO4. 

When  63.75  gm.  of  Cu  are  formed  into  CuSO4,  197,500  gm. 
calories  of  energy  are  liberated  and  when  65.37  gm.  of  Zn  are 
formed  into  ZnS04,  the  energy  liberated  is  248,000  gm.  calories. 

In  a  Daniell  cell  then,  when  65.37  gm.  of  Zn  have  been  con- 
sumed, 96,540  X  2  coulombs  have  passed  and  63.75  gm.  of  Cu 
have  been  separated  out  from  the  CuSO4,  the  energy  given  up 
by  the  cell  must  therefore  be 

248,000  -  197,500  =     50,500  gm.  calories 

=    50,500  X  4.186  watt  sec.  see  page  15 
=  211,400  watt  sec. 

and  96,540  X  2  X  voltage  of  cell  =  energy  obtained 

=  211,400  watt  sec. 

from  which  the  voltage  of  the  cell  =  =1.1  volts 

96,540  X  2 

assuming  that  no  energy  was  lost  in  the  form  of  heat. 


ART.  170]  ELECTROLYSIS  AND  BATTERIES  143 

168.  Local  Action. — In  a  well-designed  and  constructed  cell, 
action  takes  place  only  when  the  cell  is  delivering  energy.     Com- 
mercial zinc,  however,  dissolves  in  sulphuric  acid  even  when  the 
external  circuit  is  not  closed  because  the  zinc  is  impure  and  local 
action  is  set  up  between  the  impurities  and  the  zinc  and  a  number 
of  small  internal  batteries  are  formed  in  which  the  zinc  is  con- 
sumed but  no  voltage  is  available  at  the  terminals.     To  prevent 
this  action  the  zinc  is  amalgamated,  that  is  covered  with  a  layer 
of  mercury.     To  amalgamate  zinc,  clean  it  with  sandpaper,  then 
immerse  in  dilute  sulphuric  acid  and  while  still  wet  apply  mercury 
to  it  with  a  rag.     So  far  as  the  action  of  the  battery  is  concerned 
the  mercury  is  inert. 

169.  Leclanche"   Cell.— This  battery  consists  of  carbon   (C) 
and  Zinc   (Zn)  in  ammonium  chloride  (NH4C1).     The  zinc  is 
converted  into  zinc  chloride  (Zn  Cl)  wjiile  ammonia  (NH3)  and 
hydrogen  (H)  appear  at  the  carbon  plate.     To  prevent  polariza- 
tion, the  carbon  is  surrounded  with  an  oxidizing  agent  in  the  form 
of  manganese  dioxide  (Nn02),  the  oxygen  of  which  attacks  the 
hydrogen  to  form  water.     In  the  usual  form  the  carbon  is  placed 
in  a  porous  pot  and  is  surrounded  with  granules  of  manganese 
dioxide  and  of  carbon.     The  action  of  this  depolarizer  is  slow 
so  that,  when  used  to  supply  current  for  a  considerable  time,  the 
cell  becomes  polarized  and  runs  down;  it  will  recover,  however, 
if  left  on  open  circuit. 

The  battery  gives  a  voltage  of  about  1.5  on  open  circuit  and 
has  an  internal  resistance  of  about  1.5  ohms.  It  is  much  used  for 
bell  ringing  and  other  intermittent  work  and  requires  little  atten- 
tion beyond  the  addition  of  water  as  the  solution  evaporates,  and 
the  renewal  of  the  zinc  as  it  is  used  up.  The  zinc  generally  sup- 
plied is  not  amalgamated,  and  non-conducting  crystals  of  ZnCl 
stick  to  its  surface  instead  of  dropping  to  the  bottom ;  the  opera- 
tion is  improved  if  amalgamated  zinc  is  used. 

170.  Dry  Cells. — These  are  largely  used  when  the  battery  is 
subject  to  motion,  as  in  motor  cars  and  motor  boats.     There  are 
innumerable  types  but  nearly  all  consist  of  carbon,  zinc  and 
sal  ammoniac  with  other  ingredients,  in  fact  they  are  Leclanche 
cells.     The  form  usually  taken  consists  of  an  outer  cell  of  zinc 
which  is  one  electrode  and  is  lined  with  blotting  paper.     The  car- 
bon stick  is  placed  in  the  center  and  is  surrounded  with  a  mixture 
of  manganese  dioxide  and  powdered  carbon.     The  blotting  paper 
and  the  powdered  mixture  are  saturated  with  the  electrolyte  and 


144      PRINCIPLES  OF  ELECTRICAL  ENGINEERING    [CHAP,  xxn 

a  layer  of  sawdust  is  placed  on  the  top  after  which  the  cell  is 
sealed  with  pitch.  The  outside  is  then  wrapped  with  paper  so 
as  to  insulate  the  zinc.  The  current  depends  on  the  size  of  the 
cell,  the  usual  size,  2.5  in.  dia.  and  6  in.  high,  will  give  about 
15  amperes  on  short  circuit  for  a  few  seconds. 

Such  cells  deteriorate  in  storage,  to  prevent  which  one  type  has 
a  hollow  carbon  rod  with  a  stopper;  it  is  shipped  dry  and  so  is 
chemically  inactive  until  ready  for  use  when  it  is  filled  with  water 
through  the  hole  in  the  carbon. 

These  cells  run  down  as  the  active  materials  are  consumed 
but  may  often  be  recuperated  temporarily  by  the  addition  of 
sal  ammoniac  to  the  blotting  paper  through  a  hole  made  in  the 
pitch,  even  water  will  help  in  an  emergency. 

171.  Edison  Lalande  Cell. — The  active  materials  in  this  cell 
are  copper  oxide  (CuO)  and  zinc  in  a  solution  of  caustic  potash 
KOH.     The  oxygen  ions  combine  with  the  zinc  to  form  zinc 
oxide  and  this  combines  with  the  potash  to  form  a  soluble  salt. 
The  hydrogen  reduces  the  copper  oxide  to  metallic  copper  and  so 
does  not  polarize  the  cell.     The  construction  is  very  rigid.     The 
positive  plate  of  CuO  and  the  negative  Zn  plate  are  separated 
by  porcelain  spacers  and  rigidly  fastened  to  the  top  of  the  con- 
taining jar  which  jar  is  of  enameled  steel.     A  layer  of  mineral  oil 
is  placed  on  the  top  of  the  electrolyte  to  keep  out  the  air  and  to 
prevent  the  formation  of  creeping  salts. 

The  internal  resistance  of  this  cell  is  low  so  that  large  currents 
can  be  drawn  from  it;  thus  a  600  ampere-hour  battery  has  an 
open-circuit  voltage  of  0.95  and  an  internal  resistance  of  about 
0.02  ohm;  this  battery  is  designed  to  deliver  7  amp.  for  85  hours 
although  the  battery  would  give  over  40  amp.  on  short  circuit. 

172.  Power  and  Energy  of  a  Battery — The  active  materials 
in  a  battery  contain  a  definite   quantity  of    chemical  energy 
which  may  be  transformed  into  electrical  energy,  so  that  a  bat- 
tery can  give  a  definite  number  of  watt-hours  or  a  definite  num- 
ber.of  ampere-hours  at  normal  voltage. 

This  energy  may  be  taken  as  a  small  current  for  a  long  time* 
or  a  large  current  for  a  short  time  so  that  while  the  total  energy 
in  the  battery  is  fixed  by  the  weight  of  the  active  materials  the 
power  of  the  battery  (volts  X  amp.)  may  vary  over  a  wide  range. 

173.  Battery  Connections. — If  E0  is  the  open-circuit  voltage 
of  a  battery,  Rb  is  the  internal  resistance  and  R  is  the  resistance 


ART.  173] 


ELECTROLYSIS  AND  BATTERIES 


145 


of  the  external  circuit  then  the  current  7  = 

Tjl 

maximum  value  on  short  circuit  =  -°- 


and  has  a 


If  n  batteries  are  connected  in  series,  then  the  current  I  = 
~         an(*  an  mcrease  m  tne  number  of  batteries  does  not 


produce  any  considerable  increase  in  the  current  unless  R  is  large 
compared  with  Rb. 

If  n  batteries  are  connected  in  parallel,  then  the  current  /  = 

7^7 

p  i  -      D  and   an  increase  in  the  number  of  batteries  does  not 
Jtib/n  -|-  Li 

produce  any  considerable  increase  in  the  current  unless  R  is 
small  compared  with  Rb. 

The  internal  resistance  of  a  Daniell  cell  is  2  ohms  and  the  no-load  voltage 
is  1.1  volts.  The  current  when  the  batteries  are  connected  in  series  and 
in  parallel  is  given  in  the  following  table  (a)  when  the  external  resistance 
is  10  ohms  and  is  greater  than  that  of  the  battery;  (b)  when  the  external 
resistance  is  1  ohm  and  is  less  than  that  of  the  battery  : 


Number  of  cells 

Total 
resistance 
of  cells 

External 
resistance 

Amperes 

Terminal  voltage 

Open  circuit 

With 
current 

(a) 

2  ohms        10  ohms      0.092 

1.1 

0.92 

10  in  series 

20  ohms 

10  ohms 

0.365 

11.0             3.65 

10  in  parallel 

0.2  ohm 

10  ohms 

0.108 

1.1 

1.08 

(b) 

2  ohms 

1  ohm 

0.365 

1.1 

0.365 

10  in  series 

20  ohms 

1  ohm 

0.52 

11.0 

0.52 

10  in  parallel 

0.2  ohm 

1  ohm 

0.92 

1.1 

0.92 

10 


CHAPTER  XXIII 
STORAGE  BATTERIES 

174.  Action  of  the  Lead  Cell. — If  a  plate  of  lead  peroxide 
(PbC>2)  and  one  of  lead  (Pb)  are  placed  in  a  solution  of 
sulphuric  acid  (H2S04)  a  battery  is  formed  with  the  peroxide 
plate  at  the  higher  potential.  The  generally  accepted  theory  of 
operation  is  as  follows : 

The  sulphuric  acid  acts  on  the  lead  plate  to  form  lead  sulphate, 
which  material  stays  on  the  plate 

H2S04  +  Pb  =  PbSO4  +  H2 

The  hydrogen  ions  have  their  charge  neutralized  at  the  peroxide 
plate  which  they  reduce  to  lead  monoxide 

H2  +  Pb02  =  PbO  +  H2O 

The  lead  monoxide  thus  formed  is  acted  on  by  the  acid  to  form 
lead  sulphate,  which  material  stays  on  the  plate 

PbO  +  H2SO4  =  PbSO4  +  H20 

The  final  result  may  therefore  be  represented  by  the  equation 
Pb  +  Pb02  +  2H2S04  =  2PbS04  +  2H20 

so  that,  during  discharge,  sulphuric  acid  is  taken  from  the  elec- 
trolyte, water  is  added  to  it,  and  the  specific  gravity  of  the 
electrolyte  is  thereby  decreased,  while  both  plates  are  converted 
into  lead  sulphate. 

If  now  current  from  some  external  source  is  forced  through 
the  cell  in  the  opposite  direction  so  as  to  cause  electrolysis, 
the  action  is  completely  reversed : 

The  positive  H  ions  of  the  acid  are  attracted  to  the  negative 
PbSO  4  and  reduce  it  to  lead 

H2  +  PbS04  =  Pb  +  H2S04 

The  negative  SO4  ions  of  the  acid  are  attracted  to  the  positive 

146 


ART.  176]  STORAGE  BATTERIES  147 


and,  being  liberated  there,  act  on  the  water  of  the  solution 
to  form  sulphuric  acid  and'  oxygen 

2S04  +  2H20  =  2H2SO4  +  02 

This  oxygen,  with  more  water  from  the  electrolyte,  act  on  the 
sulphate  plate  to  form  peroxide  of  lead  and  sulphuric  acid 

O2  +  2H2O  +  2PbSO4  =  2PbO2  +  2H2S04 
The  final  result  may  therefore  be  represented  by  the  equation 
2PbS04  +  2H20  =  Pb  +  Pb02  +  2H2S04 

so  that  during  charge  the  plates  are  reformed  while  sulphuric 
acid  is  added  to  the  electrolyte,  water  is  taken  from  it,  and  the 
specific  gravity  of  the  electrolyte  is  thereby  increased. 

After  the  plates  have  been  completely  reformed,  further  charg- 
ing will  cause  the  hydrogen  and  oxygen  to  appear  as  gases  which 
bubble  up  through  the  electrolyte  from  the  surfaces  of  the  plates, 
the  cell  is  then  said  to  be  gassing. 

175.  Storage  or  Secondary  Battery.  —  An  electric  battery  which 
can  be  reformed  by  chemical  means  is  called  a  storage  or  sec- 
ondary  battery.     There  is  no    essential  difference  between  a 
primary  and  a   secondary  battery.     In  the  former  the  active 
materials  themselves  are  renewed  when  the  cell  is  exhausted, 
whereas  the  latter  is  designed  to  permit  of  the  materials  being 
brought  back  to  their  original  state  by  electrolysis,  that  this 
may  be  possible,  no  product  formed  during  discharge  must  be 
lost, 

176.  Sulphation.  —  There  would  appear  to  be  two  forms  of  lead 
sulphate,  an  unstable  electrolytic  form  which  is  readily  reduced 
by  an  electric  current,  and  the  lead  sulphate  formed  by  chemical 
precipitation  which  latter  is  a  non-conducting  substance  not 
decomposed  by  an  electric  current.     This  latter  substance  must 
not  be  allowed  to  accumulate  on  the  plates. 

The  electrolytic  form  changes  slowly  into  the  insoluble  form 
and  for  that  reason  a  lead  battery  must  not  be  left  discharged  for 
any  length  of  time;  insoluble  sulphate  also  tends  to  form  if  the 
battery  is  discharged  too  far. 

The  formation  of  this  insoluble  sulphate  is  called  sulphation 
and  must  be  prevented  by  proper  operation  of  the  battery.  If 
sulphation  has  commenced  on  some  of  the  plates  and  has  not 
gone  too  far,  the  plates  may  be  cleared  by  overcharging  the 


148    PRINCIPLES  OF  ELECTRICAL  ENGINEERING      [CHAP.XXIII 

battery  for  a  long  time,  the  hydrogen  and  oxygen  formed  when 
the  cell  is  overcharged  tear  off  the  insoluble  sulphate. 

177.  Construction  of  the  Plates. — When  fully  charged,  the  posi- 
tive plates  are  chocolate  in  color  and  the  peroxide  is  hard  while 
the  negative  plates  are  gray  in  color  and  the  spongy  lead  is  so  soft 
that  it  can  be  scraped  off  with  the  finger  nail.  During  discharge, 
these  plates  are  converted  into  lead  sulphate,  which  is  bulky,  so 
that  the  plates  expand  and,  unless  carefully  designed,  are  liable 
to  buckle,  especially  if  the  cell  is  discharged  rapidly  so  that  the 


FIG.  165. — Plante  plate,  showing  cross  section. 

sulphate  is  formed  rapidly  and  loosely.  The  negative  plate  in 
addition  must  be  so  designed  that  the  soft  material  will  not  be 
readily  washed  off. 

There  are  two  ways  of  forming  the  active  material.  By  the 
Plants  process  the  material  is  formed  electrochemically  out  of  the 
lead  plate  itself,  the  plates  being  grooved  or  made  in  the  form  of  a 
grill  so  as  to  have  a  large  exposed  surface.  The  plates  shown  in 
Fig.  165  are  made  out  of  pure  rolled  lead  passed  backward  and 
forward  through  grooving  rolls  which  spin  the  lead  into  ribs  the 
pitch  of  which  is  made  to  suit  the  service.  The  peroxide  and 
spongy  lead,  formed  electrochemically  on  the  positive  and  nega- 


ART.  178] 


STORAGE  BATTERIES 


149 


tive  plates  respectively,  pack  into  the  grooves  and  are  tightly  held 
in  the  narrow  spaces. 

Pasted  plates  are  made  by  spreading  a  paste  of  the  active 
material  on  to  a  supporting  grid  of  lead  hardened  with  a  small 
quantity  of  antimony,  this  being  the  only  commercially  available 
material  which  will  resist  the  action  of  sulphuric  acid  and  will  not 
set  up  local  action  with  either  the  spongy  lead  or  with  the 
peroxide.  Most  of  the  processes  by  which  these  plates  are  pre- 
pared are  secret,  but  by  such  processes  plates  can  be  made  soft 
and  porous  or  hard  and  dense  according  to  the  service  for  which 
they  are  required. 

Plante  plates  are  used  largely  for  stationary  batteries ;  they  are 
heavier  and  more  costly  than  pasted  plates  but  are  also  more 


Positive  group  Negative  group. 

FIG.  166. — Groups  of  plates  for  a  lead  battery. 

durable  and  less  liable  to  lose  active  material  by  rapid  charging 
and  discharging.  For  automobile  and  motor  truck  service, 
pasted  plates  are  generally  used  because  they  are  lighter*  than 
Plante  plates. 

178.  Construction  of  a  Lead  Battery  .—To  obtain  large  capacity 
from  a  battery,  a  large  surface  must  be  exposed  to  the  electrolyte, 
and,  since  the  size  of  a  single  plate  is  limited,  increased  capacity 
must  be  obtained  by  connecting  a  number  of  plates  in  parallel  to 
form  a  group  as  shown  in  Fig.  166,  there  being  one  more  plate  in 
the  negative  than  in  the  positive  group.  Two  sets  of  plates  are 
then  sandwiched  together,  adjoining  plates  being  separated  from 
one  another  by  glass  rods  in  the  case  of  large  power-house  cells  or 


150    I>/tI\('II>LES  OF  ELECTRICAL  ENGINEERING       [CHAP.XXIII 

by  wooden  and  rubber  separators  in  smaller  cells.  The  wooden 
separators,  see  Fig.  167,  are  specially  treated  and  are  grooved 
vertically  to  allow  the  gases  and  the  electrolyte  to  circulate  freely ; 
the  flat  side  is  placed  against  the  soft  negative  plate  while  between 
the  positive  plate  and  the  corrugations  on  the  wood  a  sheet  of 
perforated  hard  rubber  is  placed,  as  shown  in  Fig  167,  which  helps 
to  prevent  washing  out  of  the  active  material. 

The  plate  groups  are  placed  in  an  acid-proof  tank,  generally 
of  glass,  hard  rubber,  or  of  wood  lined  with  lead,  and  are  sup- 


P05!T!VE5TRAP(TYP£V} 


N£GATU5"i- 


FIG.  167.— Portable  type  of  lead  battery. 


ported  in  various  ways  as  shown  in  Figs.  168  and  169,  ample 
space  being  left  below  the  plates  for  the  accumulation  of  sedi- 
ment which  must  not  be  allowed  to  short  circuit  the  plates. 

To  minimize  leakage  of  electricity,  the  tanks  are  insulated  from 
one  another.  Small  cells  are  generally  carried  on  shallow  trays 
filled  with  sand  and  supported  on  glass  insulators  as  shown  in 
Fig.  168.  Large  lead-lined  tanks  are  generally  mounted  as  shown 
in  Fig.  169  with  a  double  set  of  insulators  between  the  tank  and 
the  ground. 


ART.  179] 


STORAGE  BATTERIES 


151 


To  prevent  loss  of  electrolyte  due  to  spraying  when  the 
cells  are  gassing  freely,  glass  sheets  are  placed  over  the  tanks. 

For  automobile  work,  hard  rubber  jars  are  used.  These  are 
placed  in  a  wooden  box  and  compound  is  poured  around  the 
jars  and  flooded  over  the  top  so  as  to  hold  them  securely  and 
also  to  seal  the  battery  and  thereby  prevent  loss  of  electrolyte. 
To  provide  for  the  escape  of  the  gases  generated  during  over- 
charge, vents  are  provided,  constructed  so  as  to  allow  the  gases 
to  escape  but  prevent  the  escape  of  the  electrolyte. 


FIG.  168. — Small  cell  in  a  glass  jar    FIG.    169. — Central  station  type  of 
supported  on  sand.  lead  cell. 

179.  Voltage  of  a  Lead  Battery. — The  terminal  voltage  of  a 
battery 

=  Eb  +  IR  while  the  battery  is  charging 
=  Eb  —  IR  while  the  battery  is  discharging 
where  Eb  is  the  internal  generated  voltage  of  the  battery 
I  is  the  current  in  amperes 
R  is  the  effective  internal  resistance  in  ohms 
so  that  the  larger  the  current  the  greater  the  difference  between 
the  charge  and  discharge  voltages. 


152     PRINCIPLES  OF  ELECTRICAL  ENGINEERING    [CHAP.XXIII 

The  value  of  Eb,  the  generated  voltage,  depends  on  the  strength 
of  the  electrolyte  and  increases  slightly  as  the  acid  becomes 
stronger,  it  therefore  increases  during  charge  and  decreases 
during  discharge. 

Tl  e  internal  resistance  of  the  cell  depends  on  the  specific 
gravity  of  the  electrolyte  and  is  a  minimum  when  this  has  a 
value  of  about  1.22,  which  is  about  the  normal  value  used  in 
cells.  During  discharge  the  acid  becomes  weak,  particularly  in 
the  pores  where  the  action  is  taking  place,  and  sulphate  is  formed 
which  is  a  comparatively  poor  conductor  of  electricity,  so  that, 
towards  the  end  of  discharge,  the  internal  resistance  of  the  cell 
increases  rapidly.  During  charge  the  acid  becomes  strong, 
particularly  in  the  pores,  so  that  toward  the  end  of  charge  the 
internal  resistance  again  rises. 


Charged  at  32  Amp. 


FIG.  170. 1— Charge  and  discharge  curves  of  the  same  lead  cell  at  different 

discharge  rates. 

The  curves  in  Fig.  170  show  how  the  voltage  varies  during 
charge  and  discharge,  the  charge  being  continued  until  the  voltage 
and  the  specific  gravity  have  become  constant  and  the  active 
materials  therefore  completely  formed,  while  the  discharge  is 
stopped  when  the  terminal  voltage  has  dropped  to  about  1.8, 
the  safe  minimum  value  at  the  normal  discharge  rate.  If  the 
discharge  is  carried  much  further  than  this  it  becomes  difficult 
to  clear  the  plates  of  sulphate  on  re-charging. 

...    average  voltage  on  discharge     Eb  —  IR  .       „    ,  , , 
The  ratio  -  ,-fe  -  =  -™— 1—7-5  iscalled  the 

average  voltage  on  charge        Eb  T  IR 

volt  efficiency  and  is  lower  the  larger  the  current  /,  that  is,  the 
higher  the  rate  of  charge  and  discharge.     At  the  normal  rates  of 
Secondary  cells  by  Aspinall  Parr;  Journal  of  the  Inst.  of  Elect.  Eng., 
vol.  36,  p.  406;  sec.  1905. 


ART.  181.]  STORAGE  BATTERIES  153 

charge  and  discharge  this  efficiency  is  seldom  less  than  80  per 
cent. 

180.  Capacity  of  a  Cell. — The  current  that  can  be  drawn  from  a 
cell  depends  on  the  plate  surface  exposed  to  the  electrolyte,  on 
the  porosity  of  the  plates,  and  on  the  rate  of  discharge.     An 
excessive  current  is  liable  to  buckle  the  plates,  see  page  148,  while 
a  short  circuit  will  cause  such  violent  action  and  such  a  sudden 
evolution  of  gas  in  the  body  of  the  active  material  as  to  cause 
parts  of  this  material  to  be  ejected  from  the  plates. 

The  capacity  of  a  battery  is  given  in  ampere-hours  at  a  definite 
rate  of  charge  and  discharge,  generally  an  8-hour  rate,  except 
in  the  case  of  automobile  batteries  which  are  generally  rated  at  a 
4-hour  rate  as  this  more  nearly  corresponds  to  the  actual  service 
conditions.  A  battery  with  a  rating  of  100  amp. -hours  will 
deliver  12.5  amp.  for  8  hours  before  the  voltage  drops  below 
1.8.  Theoretically  this  same  battery,  after  being  fully  charged, 
should  give  the  same  total  quantity  of  electricity  at  all  rates  of 
discharge,  that  is,  it  should  give  25  amp.  for  4  hours  or  50 
amp.  for  2  hours,  but  it  is  found  that  the  ampere-hour  capacity 
decreases  as  the  discharge  rate  is  increased  as  may  be  seen  from 
the  curves  in  Fig.  170.  This  particular  battery  had  a  capacity  of 

29     amp.  for  10  hours  or  290  amp. -hours  at  a  10-hour  rate, 

42.5  amp.  for    6  hours  or  255  amp. -hours  at  a    6-hour  rate, 

70     amp.  for    3  hours  or  210  amp. -hours  at  a    3-hour  rate, 

134     amp.  for    1  hour    or  134  amp.-hours  at  a    1-hour  rate. 

The  cause  of  this  loss  of  capacity  at  high  rates  of  discharge 
is  that  the  active  acid  in  the  pores  becomes  used  up  in  forming 
sulphate,  so  that  only  water  is  left  in  the  pores  since  the  acid 
is  not  replenished  fast  enough  by  diffusion  from  the  bulk  of  acid 
in  the  tank,  the  action  is  therefore  limited  to  the  surface  layers 
when  the  rate  of  discharge  is  high  and  does  not  penetrate  into 
the  active  material. 

181.  Ampere-hour  Efficiency. — Test  data  is  given  in  Fig.  170  on 
a  particular  battery  which,  after  being  completely  charged,  was 
able  to  deliver  134  amp.  for  1   hour  before  the  voltage  had 
dropped   to   1.7.     To  recharge  this  battery  required  32  amp. 
for  5  hours  or  150  amp.-hours  before  the  voltage  and  the  specific 
gravity  had  become  constant.     The  reason  for  this  additional 
quantity  of  electricity  is  that,  toward  the  end  of  a  charge,  hydrcf- 
gen  and  oxygen  gases  are  given  off  and  these  require  a  definite 


154     PRINCIPLES  OF  ELECTRICAL  ENGINEERING     [CHAP.XXHI 


quantity  of  electricity  for  their  formation,  which  electricity  is 
not  available  on  discharge  because  the  gases  have  escaped. 

If  the  charged  battery  is  now  discharged  at  42.5  amp.  the 
voltage  does  not  drop  to  1.75  until  after  6  hours  and  the  output 
is  42.5  X  6  or  255  amp. -hours.  The  action  being  slower  has 
gone  deeper  into  the  plates  and  more  active  material  has  been 
turned  into  sulphate,  but,  just  because  of  this  increased  depth  of 
action  it  is  not  advisable  to  allow  the  voltage  to  drop  so  far. 
To  recharge  this  battery  now  requires  32  amp.  for  9  hours  or 
288  amp. -hours. 

The  number  of  ampere-hours  required  to  charge  a  battery  is 
greater  than  the  number  taken  out  during  the  previous  discharge 
ampere-hours  output 


and  the  ratio 


is  called  the  am- 


ampere-hours  input  to  recharge 
pere-hour  efficiency  of  the  battery. 

From   the  test  curves  in  Fig.   170  the  following  results  are 
determined : 


Discharge 

Charge 

Amp. 

. 
1    Hours 

Amp.- 
hours 

Amp. 

Hours 

1 

Amp.- 
hours 

Amp.-hour 
efficiency, 
per  cent. 

134 

1 

134 

32 

5.1 

164 

82 

70 

3 

210 

32 

7.7 

246 

85 

42.5 

6 

255 

32 

9.0 

288 

89 

29 

10 

290 

32 

10.       I 

320 

91 

From  these  figures  it  may  be  seen  that  the  higher  the  rate  of 
discharge  the  lower  the  ampere-hour  efficiency,  the  charging 
rate  being  the  same  in  each  case.  The  reason  for  this  is  that  when 
the  discharge  rate  is  high  the  output  is  small  so  that  the  useful 
input  of  the  next  charge  is  small  and  the  portion  used  in  forming 
gases  is  proportionately  large. 

182.  Watt-hour  efficiency. — This  quantity 

_  watt-hours  output 

~~  watt-hours  input  to  recharge 

_  amp.-hours  output  X  average  discharge  voltage 

amp. -hours  input  X  average,  charging  voltage 
=  amp. -hour  efficiency  X  volt  efficiency 

both  of  which  quantities  are  lower,  the  higher  the  rate  of  charge 
and  discharge. 

With  floating  batteries,  which  are  used  to  carry  peak  loads 


ART.  184] 


STORAGE  BATTERIES 


155 


of  short  duration  and  are  not  charged  and  discharged  completely, 
there  is  little  or  no  gassing  and  the  ampere-hour  efficiency  is 
almost  100  per  cent.,  while  the  peak  voltages  at  the  end  of  a 
charge  and  the  very  low  voltages  at  the  end  of  a  discharge  are 
avoided  and  the  volt  efficiency  is  high.  Thus,  while  a  battery 
in  a  central  station  supplying  a  lighting  load  of  several  hours' 
duration  will  have  an  average  watt-hour  efficiency  over  a  period 
of  12  months  of  74  per  cent.,  a  similar  battery  in  a  central  station 
supplying  a  traction  load,  the  load  on  the  battery  being  inter- 
mittent, will  have  an  average  watt-hour  efficiency  of  84  per  cent, 
over  a  period  of  12  months. 

183.  Effect  of  Temperature  on  the  Capacity. — In  general,  the 
cooler  a  battery  is  kept  the  longer  is  its  life  but  the  lower  the 
capacity.  Lowering  the  temperature  of  the  electrolyte  increases 
its  internal  resistance  and  causes  an  increased  drop  for  a  given 
current.  The  curves  in  Fig.  171  show  how  large  is  the  drop  in 
capacity  when  the  temperature  is  lowered;  the  temperature  was 


2.0 


1.0 


10         20 


30         40 
Minutes 


50         60 


FIG.  171.* — Discharge  curves  of  a  lead  cell  with  the  same  current  but  at  differ- 
ent temperatures. 

maintained  constant  during  any  one  test  by  means  of  circulating 
water. 

The  salvation  of  a  battery  in  cold  weather  lies  in  the  fact  that 
it  is  self  warming,  the  internal  resistance  and  therefore  the 
PR  loss  become  greater  as  the  temperature  decreases.  For 
electric-truck  work  in  cold  climates  it  is  advisable  to  lag  the 
batteries,  or  at  least  to  place  them  in  a  wind-proof  compartment. 
The  temperature  on  charge  should  be  limited  to  about  40°  C.; 
continual  operation  at  higher  temperatures  tends  to  reduce 
the  life  of  the  cell.. 

184.  Limit  of  Discharge. — As  pointed  out  on  page  152,  a 
battery  should  not  be  allowed  to  discharge  to  a  voltage  below 
about  1.8  because  there  is  then  an  excess  of  sulphate  formed  on 

*Liagre,  L'Celairage  Electrique,  Vol.  29,  p.  150;  Nov.  2,  1901. 


156      PRINCIPLES  OF  ELECTRICAL  ENGINEERING   [CHAP.XXIII 


the  plates  and  a  tendency  for  the  plates  to  buckle  and  to  sulphate 
permanently.  The  amount  of  charge  in  a  battery  is  best  deter- 
mined by  measurements  of  the  specific  gravity  of  the  electrolyte 
since  this  gives  a  measure  of  the  amount  of  acid  that  has  gone 
to  form  sulphate  on  the  plates.  The  specific  gravity  may 
readily  be  measured  by  a  hydrometer  of  the  type  shown  in 
Fig.  172. 

The  voltage  of  a  battery  is  not  an  accurate  index  of  its  condition 
because  the  voltage  depends  largely  on  the  rate 
of  discharge.  Voltage  readings  on.  open  circuit 
are  of  no  value  because  this  voltage  is  almost 
independent  of  the  amount  of  charge  still  in  the 
battery. 

185.  Treatment  of  Lead  Cells. — From  a  study 
of  the  action  of  lead  cells,  the  treatment  they 
should  receive  may  be  determined.  The  manu- 
facturers' instructions  should  be  followed  in  every 
case  but  the  following  points  require  special  at- 
tention. 

A  large  part  of  the  wear  of  plates  is  due  to  gas- 
sing so  that,  while  the  beginning  of  a  charge  may 
be  at  the  2-hour  rate,  it  is  advisable  to  keep 
the  charging  rate  slow  toward  the  end  of  a 
charge;  the  average  charge  rate  of  8  hours  should 
be  used  whenever  possible. 

Cells  should   be   overcharged   about   once   a 
month  to  get  rid  of  the  last  traces  of  sulphate 
and  also  to  even  up  the  cells  and   make  sure 
that  they  are  all  charged  up  to  their  full  capacity. 
Too  rapid  discharging  causes  the  sulphate  to 
form  rapidly  and  tends  to  cause  buckling,  this 
will  seldom  cause  trouble  in  a  well-designed  bat- 
tery.   Overdischarge  however  must  be  avoided,  the  terminal  volt- 
age not  being  allowed  to  drop  below  1.8  at  the  normal  8-hour 
rate  nor  below  1.75  at  the  4-hour  discharge  rate. 

The  cell  should  not  be  allowed  to  stand  discharged  for  any 
length  of  time.  If  a  battery  has  to  stand  idle  for  several  months  it 
should  be  charged  monthly  because  even  a  small  leakage  current 
will  cause  enough  sulphate  to  form  on  the  plates  to  cause  trouble 
if  it  turns  into  the  insoluble  form.  This  monthly  charge  should 
be  continued  until  there  is  no  further  rise  in  voltage  or  of  specific 


FIG.  172— Hy- 
drometer  for 
testing  the  spe- 
cific gravity  of 
the  electrolvte. 


ART.  186]-  STORAGE  BATTERIES  157 

gravity  and  until  the  cell  has  been  gassing  for  about  5  hours,  one 
may  then  be  reasonably  sure  that  no  sulphate  has  been  left  on  the 
plates. 

Evaporation  of  the  electrolyte  should  be  made  good  by  the 
addition  of  pure  water;  the  acid  does  not  evaporate  and,  unless 
there  is  excessive  spraying  due  to  the  gases  given  off,  the  quantity 
of  acid  in  the  cell  will  not  change. 

The  specific  gravity  of  the  acid  used  depends  on  the  use  to 
which  the  cell  will  be  put  while  the  permissible  change  in  the 
specific  gravity  depends  on  the  bulk  of  acid  in  the  cell  and 
should  be  obtained  from  the  maker.  When  the  cell  has  to  stand 
inactive  for  long  periods,  a  weak  acid  is  used  to  lessen  the  risk  of 
sulphation.  For  automobile  work,  the  quantity  of  acid  in  the 
cell  should  be  such  that  the  specific  gravity  shall  not  change  more 
than  from  1.28  to  1.17  between  full  charge  and  full  discharge;  for 
other  service  a  range  of  from  1.23  to  1.15  is  more  usual,  the  gravity 
being  measured  at  a  temperature  of  70°  F. 

Before  removing  sediment  from  a  cell,  the  plates  should  be 
fully  charged,  then  taken  out  and  the  separators  removed.  The 
plates  and  tank  should  then  be  washed  with  water  and  the  whole 
battery  put  back  into  commission  before  the  plates  have  time  to 
dry. 

The  gases  formed  during  overcharge  are  explosive  so  that  a 
naked  flame  should  be  kept  away  from  the  battery  room.  The 
room  also  should  be  well  ventilated  and  the  floor  and  walls  should 
be  of  some  acid-resisting  material  such  as  vitrified  brick.  The 
room  should  be  obscured  from  direct  sunlight,  which  tends  to 
warp  the  plates;  a  heating  system  should  be  put  in  if  the  battery 
has  to  operate  in  a  cold  climate,  so  as  to  maintain  the  capacity 
under  all  conditions. 

186.  Action  of  the  Edison  Battery. — If  a  plate  of  nickel  oxide 
(Ni02)  and  one  of  iron  (Fe)  are  placed  in  a  solution  of  caustic 
potash  (KOH),  a  battery  is  formed  with  the  nickel  oxide  plate 
at  the  higher  potential. 

If  current  is  drawn  from  this  battery,  the  oxide  (Ni02)  is  re- 
duced to  a  lower  oxide  (Ni203)  while  the  iron  is  oxidized  to  form 
FeO,  and  the  cell  is  gradually  discharged; 

2Ni02  +  Fe  =  Ni203  +  FeQ 

If  now  current  from  some  external  source  is  forced  through 
the  cell  in  the  opposite  direction  so  as  to  cause  electrolysis,  the 


158    PRINCIPLES  OF  ELECTRICAL  ENGINEERING      [CHAP.XXIII 

action  is  completely  reversed.  The  negative  O  ions  are  attracted 
to  the  positive  Ni203  and  the  higher  oxide  NiO2  is  reformed  while 
the  positive  H  ions  are  attracted  to  the  negative  FeO  and  reduce  it 
to  iron 

Ni203  +  FeO  =  2NiO2  +  Fe 

the  result  of  charge  and  discharge  is  a  transfer  of  oxygen  from  one 
plate  to  the  other;  the  strength  of  the  electrolyte  is  not  changed 
so  that  the  quantity  required  is  less  than  for  an  equivalent  lead 
cell. 

After  the  battery  has  been  completely  charged,  the  hydrogen 
and  oxygen  appear  as  gases  which  bubble  up  through  the 
electrolyte  just  as  in  the  lead  cell. 

187.  Construction  of  the  Plates. — The  positive  or  nickel  plate 
shown  in  Fig.  173  consists  of  a  nickel-plated  steel  grid  carrying 
perforated  steel  tubes,  one  of  which  is  shown  in  diagram  B .     These 
tubes  are  heavily  nickel  plated  and  are  filled  with  alternate  layers 
of  nickel  hydroxide  and  flaked  metallic  nickel.     The  hydroxide  is 
acted  on  electrochemically  and  becomes  nickel  oxide.     This  oxide  is 
such  a  poor  conductor  of  electricity  that  the  flaked  nickel  is  added 
to  bring  the  inner  portions  of  the  oxide  into  metallic  contact  with 
the  surface  of  the  tubes  and  thereby  reduce  the  internal  resistance 
of  the  cell. 

Each  tube  has  a  lapped  spiral  seam  to  allow  for  expansion,  and 
is  reenforced  with  steel  rings  to  prevent  the  tube  from  expand- 
ing away  from  and  breaking  contact  with  the  enclosed  active 
material. 

The  negative  or  iron  plate  shown  in  Fig.  173  consists  of  a  nickel 
plated  steel  grid  holding  a  number  of  rectangular  pockets  filled 
with  powdered  iron  oxide.  Each  pocket  is  made  of  two  pieces  of 
perforated  steel  ribbon  flanged  at  the  side  to  form  a  little  flat  box 
which  may  be  filled  from  the  end. 

188.  Construction  of  an  Edison  Battery. — A  number  of  like  plates 
are  connected  in  parallel  to  form  a  group,  there  being  one  more 
plate  in  the  negative  than  in  the  positive  group.     Two  sets  of 
plates  are  then  sandwiched  together  as  shown  in  Fig.  174,  ad- 
joining plates  being  separated  from  one  another  by  strips  of  hard 
rubber.     End  insulators  A   are  provided  with  grooves  which 
carry  the  edges  of  the  plates,  and  thereby  act  as  spacers  and  at 
the  same  time  insulate  the  plates  from  the  steel  tank.     The  out- 
side negative  plates  are  insulated  from  the  tank  by  sheets  of  hard 


ART.  188] 


STORAGE  BATTERIES 


159 


rubber,  while  the  whole  unit  rests  on  the  rubber  rack  B  by  which 
the  plates  are  insulated  from  the  bottom  of  the  tank;  this  rack  is 
shallow  since  very  little  space  is  required  for  sediment  in  an 
Edison  cell. 


A.  Pocket  for  negative  plate. 


B.  Tube  for  the  positive  plate. 


Positive  plate  Negative  plate. 

FIG.  173. — Plates  of  an  Edison  Battery. 

The  tank,  which  is  made  of  cold  rolled  steel  welded  at  the  joints, 
is  corrugated  for  strength  as  shown  in  Fig.  175  and  is  heavily 
nickel  plated  as  a  protection  against  rust.  The  cover  is  of  the 


160      PRINCIPLES  OF  ELECTRICAL  ENGINEERING  [CHAP,  xxm 


same  material  and  is  welded  to  the  rest  of  the  tank  after  the  plates 
have  been  put  in  place.  This  cover  carries  two  terminals,  as 
well  as  a  combined  gas  vent  and  filling  aperture  A.  When  the 
cover  b  is  closed,  the  hemispherical -valve  a  closes  the  aperture 
and  prevents  the  escape  of  electrolyte,  but  allows  the  gases 
generated  on  overcharge  to  escape  as  soon  as  the  pressure  in 
the  tank  becomes  high  enough  to  raise  the  valve. 

The  electrolyte  used  consists  of  a  21  per  cent,  solution    of 
potash  in  distilled  water  to  which  a  small  amount  of  lithia  is 


FIG.  174. — Plate  groups  of  an 
Edison  cell. 


FIG.  175. — Top  of  the  jar  of  an  Edi- 
son cell. 


added.     No  corrosive  fumes  are  given  off  from  this  electrolyte 
so  that  no  special  care  need  be  taken  in  mounting  the  cells. 

189.  The  Voltage  of  an  Edison  Battery. — Fig.  176  shows  how 
the  voltage  of  an  Edison  battery  changes  when  the  battery  is 
charged  and  then  discharged.     The  voltage  characteristics  are 
similar  to  those  of  a  lead  battery. 

There  is  no  lower  limit  to  the  voltage  of  an  Edison  battery 
because  in  it  there  is  nothing  eqivalent  to  sulphation,  but  dis- 
charge is  not  continued  below  a  useful  lower  limit. 

190.  Characteristics   of  an  Edison  Battery. — These  batteries 
are  rated  at  a  7-hour  charging  rate  and  a  5-hour  discharge  rate 


ART.  190.1 


STORAGE  BATTERIES 


161 


with  the  same  current  in  each  case,  the  ampere-hour  efficiency 
being  about  82  per  cent,  at  this  rate  and  the  internal  heating 
not  more  than  permissible.  A  higher  rate  of  discharge  may  be 
used  so  long  as  the  internal  temperature  does  not  exceed  about 
45°  C.;  continual  operation  at  higher  temperatures  shortens 
the  life  -of  the  cell.  A  longer  charge  rate  than  7  hours  should 
not  be  used  because,  with  low  currents,  the  iron  element  is  not 
completely  reduced;  this  however  does  not  permanently  injure 
the  cell  but  makes  it  necessary  to  overcharge  the  cell  at  normal 
rate  and  then  discharge  it  completely  to  bring  it  back  to  normal 
condition. 

Because  of  the  comparatively  high  internal  resistance  of  the 


FIG.  176. — Charge  and  discharge  curves  of  a  lead  cell  and  an  Edison  cell. 

Edison  battery,  the  volt  efficiency  is  lower  than  in  the  lead  cell, 
as  may  readily  be  seen  from  Fig.  176,  and,  since  the  ampere- 
hour  efficiency  is  not  any  higher,  the  watt-hour  efficiency  of  the 
Edison  cell  is  also  lower. 

The  great  advantages  of  the  Edison  cell  are  that  it  is  lighter  than 
the  lead  cell  and  is  more  robust,  it  can  remain  charged  or  dis- 
charged for  any  length  of  time  without  injury,  and  so  little  sedi- 
ment is  formed  that  the  makers  seal  it  up.  Since  no  acid  fumes 
are  given  off,  the  cell  may  be  placed  in  the  same  room  as  other 
machinery  without  risk  of  corrosion  of  that  machinery. 

The  chief  disadvantage  of  the  Edison  cell,  in  addition  to  its 
high  cost,  is  that  its  efficiency  is  lower  than  that  of  the  lead  cell. 


11 


CHAPTER  XXIV 


OPERATION  OF  GENERATORS 

191.  Operation  of  the  Same  Shunt  Machine  as  a  Generator  or 
as  a  Motor. — The  generator  G,  Fig.  177,  driven  in  the  direction 
shown,  supplies  power  to  the  mains  mn.  The  same  machine, 
operating  as  a  motor  from  mains  of  the  same  voltage  and  polarity, 
is  shown  at  M;  the  direction  and  the  strength  of  the  shunt  field 
are  unchanged,  the  direction  of  the  armature  current  is  reversed,  but 

-lii L 


Generator  Motor 

FIG.  177. — Operation  of  the  same  shunt  machine  as  a  generator  and  as  a 

motor. 

the  direction  of  motion,  determined  by  the  left-hand  rule,  page  7 
is  the  same  as  in  G.  Since  the  back  e.m.f.  when  the  machine  is 
operating  as  a  motor  has  to  be  practically  equal  to  E,  the  e.m.f. 
of  the  machine  when  operating  as  a  generator,  the  machine  must 
run  at  the  same  speed  in  each  case. 

In  diagram  A,  Fig.  178,  m  and  n  are  two  mains  kept  at  a 
constant  voltage  E  by  the  generators  in  a  power  house,  and  D 
is  a  single  shunt  generator  of  the  same  voltage  running  at  normal 
speed.  If  the  voltage  Ed  is  exactly  equal  to  E  and  the  polarity 

162 


ART.   192] 


OPERATION  OF  GENERATORS 


163 


is  as  shown,  then  no  current  will  flow  in  the  lines  a  and  b  when 
the  switch  S  is  closed.  If  the  excitation  of  D  is  now  increased 
slightly  so  that  the  voltage  generated  in  the  machine  is  greater 
than  the  line  voltage  E,  then  current  will  flow  in  the  direction 
of  the  generated  voltage,  as  shown  in  diagram  B,  and  the  machine, 
operating  as  a  generator,  will  supply  power  to  the  circuit  mn. 
If  now  the  excitation  of  D  is  decreased  so  that  the  voltage  gen- 
erated in  the  machine  is  less  than  the  line  voltage  E,  then  current 
will  flow  in  the  direction  of  the  greater  voltage,  that  is,  in  a 
direction  opposite  to  that  of  the  generated  voltage,  as  in  diagram 
C,  and  the  machine,  operating  as  a  motor  in  the  same  direction 
as  before,  will  take  power  from  the  circuit  mn.  Thus  by  merely 
varying  the  excitation  of  D  it  may  be  made  to  act  as  a  generator 
or  as  a  motor. 


FIG.  178. — Operation  of  the  same  shunt  machine  as  a  generator  and  as  a 

motor. 

192.  Loading  Back  Tests. — Load  tests  on  large  electrical  ma- 
chines must  be  made  by  some  method  whereby  the  power  de- 
veloped by  the  machine  is  not  dissipated  but  is  made  available 
for  the  test,  otherwise  the  power-house  capacity  may  not  be 
large  enough  to  allow  many  machines  to  be  tested,  while  the  cost 
of  such  tests  will  be  excessive. 

If  the  machine  to  be  tested  is  a  generator,  it  is  driven  at  normal 
speed  by  a  motor  of  the  same  voltage  but  of  larger  capacity,  and 
both  machines  are  connected  to  the  power  house  mains  as 
shown  in  Fig.  179.  The  motor  M  is  started  up  by  means  of  a 
starting  box  in  the  usual  way,  and  the  generator  G  is  excited  until 
its  voltage  is  equal  to  E,  the  switch  Si  is  then  closed  and  a  volt- 
meter V  is  placed  across  the  switch  S*.  If  the  reading  of  this 
voltmeter  is  twice  normal  voltage,  then  the  polarity  of  G  must  be 


164     PRINCIPLES  OF  ELECTRICAL  ENGINEERING    [CHAP,  xxiv 


reversed  by  reversing  the  shunt  field,  but  if  the  reading  is  zero,  then 
the  switch  82  may  be  closed  and  the  generator  G  thereby  con- 
nected to  the  mains.  There  will  then  be  no  current  in  the  leads 
a  and  b  so  that  the  generator  will  be  running  light,  and  the  motor 
will  be  taking  from  the  power  house  only  that  power  required  to 
operate  the  two  machines  at  no-load. 

If  the  excitation  of  G  is  now  increased  so  as  to  increase  its 
generated  voltage,  then  current  will  flow  in  the  direction  shown; 
the  machine  will  deliver  power  to  the  circuit  mn  while  the  motor 
M  ,  which  drives  (7,  will  take^from  this  same  circuit  an  amount  of 
power  equal  to  the  output  of  G  plus  the  losses  in  the  two  machines, 
of  which  the  portion  required  to  supply  the  losses  is  all  that  is 

taken  from  the  power  house  since 
the  output  of  G  is  sent  back  into  the 
power  mains. 

193.  Parallel  Operation.—  The 
load  on  a  power  station  is  generally 
distributed  among  several  genera- 
tors connected  in  parallel  with  one 
another,  so  that  a  breakdown  of 
one  unit  will  not  seriously  cripple 
the  station.  Parallel  operation  of 
generators  has  the  additional  ad- 


To Power-house 


Si    S, 


FIG.  179. — Loading  back  test  on 
a  generator. 


vantage  that  the  number  of  generating  units  in  operation  can  be 
changed  with  the  load,  so  as  to  maintain  the  individual  machines 
at  approximately  full-load,  at  or  near  which  load  they  operate 
with  their  highest  efficiency. 

194.  Shunt  Generators  in  Parallel.—  A  and  B,  Fig.  180,  are 
shunt  generators  which  feed  into  the  same  mains  m  and  n.  Sup- 
pose that  A  has  been  carrying  all  the  load  and  that  it  has  become 
necessary  to  connect  generator  B  to  the  mains  to  share  the  load. 
This  latter  machine  is  brought  up  to  speed  with  the  switch  S  open, 
its  field  rheostat  is  adjusted  until  Eb  is  equal  to  E,  and  the  switch 
S  is  closed.  The  load  on  B  is  then  zero.  To  make  the  two 
machines  divide  the  load,  the  excitation  of  B  is  increased  so  as  to 
increase  its  generated  voltage  and  thereby  cause  the  machine  to 
deliver  current  to  the  mains. 

If,  due  to  a  momentary  increase  in  speed  or  for  some  other  reason, 
machine  A  takes  more  than  its  proper  share  of  the  total  load,  the 
voltage  of  A  drops  since  it  is  a  shunt  generator,  see  page  73,  and 
part  of  this  load  is  automatically  thrown  on  B,  the  machine  with 


ART.  195] 


OPERATION  OF  GENERATORS 


165 


E 


\ 


the  higher  voltage  at  that  instant.  Furthermore,  if  the  engine 
connected  to  B  fails  for  an  instant,  that  machine  slows  down,  its 
generated  voltage  drops  and  the  load  is  automatically  thrown  on 
A ;  if  this  generated  voltage  drops  far  enough,  then  current  flows 
from  the  line  to  operate  machine  B  as  a  motor  at  normal  speed  and 
in  the  same  direction  as  before,  see 
page  162,  but,  as  soon  as  the  engine 
recovers,  this  machine  again  takes 
its  share  of  the  load.  The  opera- 
tion of  two  shunt  machines  in  paral- 
lel is  therefore  stable,  each  machine 
refuses  to  take  more  than  its  proper 
share  of  the  load  and  yet  helps  the 
other  machine  when  necessary. 

To  disconnect  machine  B,  its  ex- 
citation should  be  reduced  until  A 
is  carrying  all  the  load,  the  switches 
S  may  then  be  opened. 

195.   Division    of    Load    among 

Shunt  Generators  in  Parallel. — The  external  characteristics  of 
the  two  shunt  generators  are  shown  in  Fig.  181.  When  the  line 
voltage  is  E,  the  currents  in  the  machines  are  Ia  and  Ib  and  the 
line  current  is  Ia  +  Ib.  If  the  current  drawn  from  the  mains 


FIG..  180.^-Parallel  operation  of 
shunt  generators. 


^SSK 

=5= 

E  ' 

^       | 

^      f 

"o 

'o 

"3 

— 

^ 

q 

B 

1 
& 

1 

1' 

H 

H 

ia    ib 

la 

*b 

Ia 

h 

Armature  Current 


Armature  Current 


100 


400 
Kilowatts 


FIG.  181  FIG.  182  FIG.  183 

FIGS. — 181,  182  and  183. — Division  of  load  between  two  shunt  generators  in 

parallel. 

decreases,  the  voltage  E  rises  and  the  currents  in  the  two  ma- 
chines are  then  ia  and  ib  when  the  line  current  is  ia  +  4- 

To  make  machine  A  take  a  larger  portion  of  the  total  load,  its 
excitation  must  be  raised  so  as  to  raise  its  characteristic  as  shown 
in  Fig.  182. 

If  a  100-kw.  and  a  400-kw.  machine  have  the  same  regulation 


166     PRINCIPLES  OF  ELECTRICAL  ENGINEERING    [CHAP,  xxiv 

and  therefore  the  same  drop  in  voltage  from  no-load  to  full- 
load  then,  as  shown  in  Fig.  183,  the  machines  will  divide  the 
load  according  to  their  respective  capacities. 

196.  Compound  Generators  in  Parallel. — A  and  5,  Fig.  184, 
are  two  compound  generators  which  are  operating  in  parallel. 
If,  due  to  a  momentary  increase  in  speed,  machine  A  takes 
more  than  its  proper  share  of  the  total  load,  the  series  excita- 
tion of  A  increases,  its  voltage  rises,  and  it  takes  still  more  of 
the  load,  so  that  the  operation  is  unstable. 

To  prevent  this  instability,  the  points  e  and  /,  Fig.  185,  are 
joined  by  a  connection  of  large  cross  section  and  of  negligible 
resistance,  called  an  equalizer  connection.  The  series  coils  P 
and  Q  are  thus  connected  in  parallel  with  one  another  between 
the  equalizer  and  the  negative  main  n,  as  is  shown  more  clearly 


Equalizer 


b    c 


FIG.    184. — Compound  generators 
in  parallel. 


FIG.  185. — Compound  generators 
in  parallel,  an  equalizer  being  sup- 
plied and  the  machine  A  being 
over-loaded. 


in  Fig.  186,  and  the  total  current  from  the  negative  main  n 
always  passes  through  these  coils  in  one  direction  and  divides 
up  between  them  inversely  as  their  resistance,  independently  of 
the  distribution  of  the  load  between  the  machines.  If  now,  due 
to  a  momentary  increase  in  speed,  machine  A  takes  more  than 
its  proper  share  of  the  total  load,  as  shown  in  Fig.  185,  and 
therefore  less  is  left  for  machine  B,  the  series  excitation  of  the 
two  machines  is  unchanged,  since  the  total  load  is  unchanged, 
so  that  the  machines  act  as  shunt  generators  with  a  constant 
superimposed  excitation  and  the  voltage  of  A  decreases  and  that 
of  B  increases,  and  part  of  this  load  is  automatically  thrown 
on  B,  the  machine  with  the  higher  voltage;  the  operation  of  the 
machines  has  therefore  been  made  stable  by  the  addition  of  the 
equalizer  connection. 


ART.  197J 


OPERATION  OF  GENERATORS 


167 


To  connect  machine  B  in  parallel  with  machine  A  which  is 
already  running,  bring  the  machine  up  to  speed  with  the  switches 
a,  b,  and  c  open,  close  switches  b  and  c  in  order  to  excite  the  series 
coils,  then  adjust  the  shunt  excitation  until  Eb  is  equal  to  E, 
and  finally  close  switch  a,  the  machine  may  then  be  made  to  take 
its  share  of  the  load  by  increasing  its  shunt  excitation.  To 
disconnect  the  machine,  its  shunt  excitation  should  be  reduced 
until  all  the  load  has  been  transferred  to  A,  the  switches  should 
then  be  opened  in  the  reverse  order. 

For  large  machines  three  separate  switches  are  generally  used. 
For  smaller  machines  the  switches  b  and  c  are  often  combined 
to  form  a  double  pole  switch.  When  the  machines  are  a  con- 
siderable distance  from  the  mains  m  and  n,  the  equalizer  is  often 
run  straight  between  the  machines  as  shown  in  Fig.  186. 


Equalizer  L 


FIG.  186. — Compound  generators  in  parallel,  showing  methods  of  changing 

the  compounding. 

197.  Division  of  Load  among  Compound  Generators. — When  a 
single  compound  generator  has  too  much  compounding,  a  shunt 
in  parallel  with  the  series  field  coils  will  reduce  the  current  in  these 
coils  and  so  reduce  the  compounding,  page  77. 

When  one  of  a  number  of  compound  generators  in  parallel 
is  found  to  take  more  than  its  share  of  the  load,  then  its  com- 
pounding must  be  reduced,  this,  however,  can  no  longer  be  ac- 
complished by  placing  a  shunt  in  parallel  with  the  series  coils  of 
that  machine,  for  example  the  shunt  S  will  not  only  reduce  the 
current  in  the  series  coils  Q  but  will.at  the  same  time  reduce  the 
current  in  the  series  coils  P  since  the  two  sets  of  series  coils  and 
the  shunt  S  are  then  all  connected  in  parallel  between  the  negative 
main  and  the  equalizer,  as  shown  in  diagram  X,  and  the  total 
line  current  will  divide  among  them  inversely  as  their  resist- 


168     PRINCIPLES  OF  ELECTRICAL  ENGINEERING    [CHAP,  xxiv 

ance.     The  compounding  of  both   machines   will   therefore  be 
reduced. 

To  reduce  the  current  in  the  series  coils  Q  without  at  the  same 
time  reducing  that  in  the  coils  P,  a  resistance  must  be  placed  in 
series  with  Q  as  shown  in  diagram  Y. 


CHAPTER  XXV 

OPERATION  OF  GENERATORS  AND  BATTERIES  IN 
PARALLEL 

198.  Isolated   Lighting   Plants. — The   engine   and   generator 
capacity   in   such   plants   is  generally   sufficient   for   the  total 
number  of  lamps  connected,  but  since  the  lamps  are  seldom  all 
in  use  at  one  time,  the  plant  operates  at  partial  load  and  con- 
sequently with  low  efficiency.     When  a  suitable  storage  battery 
is  installed,  the  generator  may  be  operated  for  a  few  hours  to 
charge  the  battery  and  may  then  be  shut  down,  the  battery 
being  left  connected  to  supply  the  load  current. 

199.  Lighting  Plants  for  Farm  Houses. — The  equipment  for 
such    plants  is  shown    diagrammatically  in  Fig.    187,   30-volt 
tungsten  lamps  being  used  since  they  have  stronger  filaments 
than  110- volt  lamps  and  can  therefore  be  made  in  smaller  sizes, 
see  page  353. 

The  voltage  of  a  lead  cell  varies  from  about  2.65  volts  on  full 

charge  to  2.2  volts  at  the  beginning  of  discharge  and  1.8  volts 

at  the  end  of  discharge,  so  that  if  16  cells  are  connected  in  series 

then:  the  battery  voltage  on  full  charge  =  16  X  2.65  =  42.5  volts, 

at  the  beginning  of  discharge         =  16  X  2.2    =35.2  volts, 

at  the  end  of  discharge  =  16  X  1.8    =  28.8  volts. 

If  the  voltage  across  the  lamps  is  raised  much  above  35  volts, 
then  the  30-volt  lamps  will  be  burnt  out,  so  that  the  lamp  circuit 
must  be  disconnected  while  the  generator  is  charging  the  battery. 

Specify  the  generator  and  battery  for  a  lighting  plant  with  a  connected 
load  of  24  tungsten  lamps  of  15  watts  and  12  candle  power  each, 
watts  per  lamp  =15 

current  per  lamp         =  15/30  =  0.5  amp. 
current  for  24  lamps  =  0.5  X  24  =  12  amp. 

battery  capacity  at  normal  8-hour  rate  =  12  X  8  =  96  amp. -hours 
maximum  generator  voltage  =  16  cells  at  2.65  volts  =  42.5  volts 
charging  current  at  8- hour  rate  =  12  amp. 
generator  output  =  12  X  42.5  =  510  watts 

169 


170       PRINCIPLE*  OF  ELECTRICAL  ENGINEERING     [CHAP,  xxv 

If  the  average  daily  load  on  the  battery  is  6  lamps  for  4  hours  then  the 
ampere-hours  taken  each  day  =  (6  X  0.5)  X  4  =  12  and  a  96-amp  .-hour 
battery  will  supply  this  load  for  96/12  or  8  days  without  having  to  be  re- 
charged. 

To  prevent  the  generator  from  being  connected  in  parallel 
with  the  battery  except  when  its  voltage  is  higher  than  the 
battery  voltage,  an  automatic  switch  S  is  supplied.  To  charge 
the  battery,  the  lights  are  disconnected,  the  generator  which 
is  shunt  wound  is  started  up  by  the  engine,  and  the  shunt 
rheostat  r  is  gradually  cut  out  to  raise  the  generator  voltage 
until  the  value  is  reached  for  which  the  solenoid  F  was  set,  the 
pull  of  this  solenoid  then  closes  the  switch  S  and  connects  the 
generator  in  parallel  with  the  battery.  The  generator  then 
delivers  current  to  the  battery,  which  current  flowing  through 
the  coil  H,  adds  to  the  pull  of  F  and  helps  to  keep  the  switch  S 
closed. 


L 


Lamps 


Carbon  Pile 


FIG.  187. — Connection  diagram  for  a  small  iso-      FIG.    188. — Lamp  circuit 
lated  lighting  plant.  regulator. 

If,  due  to  a  loose  field  connection  or  for  some  other  reason,  the 
generator  voltage  drops  below  that  of  the  battery  then  current 
flows  back  through  the  coil  H  in  such  a  direction  as  to  oppose  the 
pull  of  F'}  the  switch  S  is  thereby  released  and  the  generator  dis- 
connected from  the  circuit.  The  switch  S  therefore  acts  as  a 
reverse  current  circuit  breaker. 

200.  Lamp  Circuit  Regulator.— One  objection  to  the  above 
system  is  that  the  voltage  across  the  lamps  varies  with  the 
battery  voltage  from  35.2  to  28.8  and  the  life  of  the  lamps  is 
shortened  due  to  the  high  voltage  while  the  lighting  is  unsatis- 
factory when  the  voltage  is  below  30  volts.  This  trouble  may 
be  overcome  by  the  addition  of  an  automatic  regulator.  A  very 
simple  type  of  regulator  for  this  purpose  is  shown  diagram- 
matically  in  Fig.  188  and  consists  of  a  carbon  pile  resistance 


ART.  202]  OPERA  TION  OF  GENERA  TORS  A  NO  BA  TTERIEH         1 7 1 


R  inserted  in  the  lighting  circuit,  and  a  shunt  solenoid  P  by  which 
the  pressure  on  the  carbon  pile  is  varied.  If  the  voltage  across 
the  lamp  circuit  increases,  the  current  in  the  solenoid  P  increases 
and  lifts  the  lever  L,  thereby  reducing  the  pressure  on  the 
carbon  pile  R  and  increasing  its  resistance,  so  that  the  voltage 
drop  across  the  carbon  pile  increases  and  the  lamp  voltage  re- 
mains approximately  constant.  A  more  elaborate  regulator 
of  ,this  type  is  described  on  page  182. 

201.  Small    Isolated    Power    Stations. — In    such    stations, 
provision  must  be  made  for  charging  the  battery  and  also  for 
carrying  the  day  load  at  the  same  time.     This  is  accomplished 
either  by  resistance  control,  end  cell  control  or  booster  control. 

202.  Resistance  Control. — To  take  the  case  of  a  110-volt  plant. 
The  number  of  cells  in  series  is  110/1.8  =  60  and,  to  charge 
them  in  series,  would  require  a  maximum  voltage  of  60  X  2.65  = 


Si 


Si 

30  Volts 
_i_ 

,  c 

p=ccz]  S                           J 

i  :  

J 

Generator 

Overload- 
Circuit 
Breaker 

0 

-x 
Overlc 
Curreu 

j 

>ada 
Gil 

Load 

110  Volts 

_l± 


H'l'l' hhlhr 


!so  Volts; 


Y 

A-  Charging 

"Mil.!.  .          i.l.j  .• 


B-  Discharging 


FIG.  189. — Resistance  system  of  battery  control. 

160  volts.  But  since  the  day  load  has  to  be  carried  by  the 
generators  while  the  battery  is  being  charged,  it  is  not  permis- 
sible to  raise  the  generator  voltage  above  110  volts.  This 
difficulty  is  overcome  by  dividing  the  battery  into  two  halves 
for  charging  purposes  and  connecting  them  to  the  generator 
as  shown  in  Fig.  189,  the  maximum  battery  voltage  during  charge 
will  then  be  80  volts  and  the  remaining  part  of  the  110  volts  must 
be  used  up  in  the  resistance  R}  by  means  of  which  resistance 
the  charging  current  may  be  regulated. 

When  the  battery  is  fully  charged  the  cells  are  reconnected 
in  series  by  throwing  the  switches  over  into  position  Si  and, 
since  the  battery  voltage  at  the  beginning  of  discharge  is 


172      PRINCIPLES  OF  ELECTRICAL  ENGINEERING    [CHAP,  xxv 

60  X  2.2  or  132  volts  while  the  generator  voltage  is  only  110  volts, 
the  same  resistance  R  must  be  kept  in  the  battery  circuit  to 
control  the  discharge. 

203.  End  Cell  Control.— With  this -system  of  control,  shown 
diagrammatically  in  Fig.  190,  the  battery  is  charged  in  series  from 
a  generator  which  has  a  voltage  range  up  to  160  volts,  the  charg- 
ing current  being  regulated  by  the  generator  field  rheostat.  Two 
end  cell  switches  Ci  and  C2  are  required  so  that  the  110-volt  load 
may  be  supplied  while  the  battery  is  being  charged. 

When  the  battery  is  at  the  end  of  discharge,  the  number  of 


_J 

—  eSl  fc       Acq 

s 

—  sSii 

s 

^=3. 

1 

Overload 
and 
Reverse 
"Current 

tl  ' 

c.b. 

*  160-> 
Volts 

o 

Overl 
C.br 

dd 

180      j             110  volts 
V°ltsj     onLoadCivcuit 

R 

i^B 

L 

Ci    + 

_ 

_      110  Volts                Load 

u 

HI 


-0 
H3 

C-  End  Cell 
Switch 


I  110  Volts 


A- Charging  B  -discharging 

FIG.  190. — End  cell  system  of  battery  control. 

cells  for  110  volts  is  110/1.8  or  61  while  at  the  beginning  of 
discharge  110/2.2  or  50  cells  are  all  that  are  required,  and  11  end 
cells  must  be  so  arranged  that  they  can  be  gradually  connected 
in  circuit  as  the  battery  discharges.  When  the  end  cell  switches 
are  in  the  position  shown  in  Fig.  190,  there  are  50  main  cells 
and  4  end  cells  on  the  lighting  circuit  while  all  the  cells  are  being 
charged  in  series;  these  4  cells  on  the  lighting  circuit  will 
gradually  be  cut  out  as  the  battery  becomes  more  fully  charged 
and  the  voltage  rises. 

When  the  battery  is  fully  charged,  the  switches  are  thrown  over 


ART.  204]  OPERA  TION  OF  GENERA  TORS  AND  BATTERIES        1 73 


into  the  position  Si  and  the  generator  and  battery  are  thereby 
connected  in  parallel  across  the  mains. 

In  order  that  the  end  cell  switch  will  not  open  the  circuit  when 
passing  from  one  contact  to  that  adjoining  nor  yet  will  it  short 
circuit  any  one  cell,  this  switch  is  generally  constructed  as  shown 
diagrammatically  in  Fig.  190  with  a  main  contact  a  and  an  aux- 
iliary contact  b  electrically  connected  through  a  resistance  r  but 
otherwise  insulated  from  one  another.  As  the  switch  is  moved 
over  the  contacts,  it  stands  for  an  instant  in  the  position  shown 
and  bridges  one  cell,  but  the  resistance  r  keeps  the  current  that 
flows  through  this  cell  from  being  dangerously  large. 

Since  the  end  cells  are  gradually  put  in  circuit  as  discharge 
proceeds,  they  are  never  so  completely  discharged  as  the  rest  of 
the  battery,  so  that,  when  the  battery  is  being  recharged  with  all 
the  cells  in  series,  the  end  cells  should  be  cut  out  one  by  one  as 
they  become  fully  charged  and  begin  to  gas  freely. 


!     AsOVoHs 

i  rr 


A    Battery  Charging 


1 110  Volts 


B    Battery  Discharging 

FIG.  191. — Booster  charge  and  end  cell  discharge  system  of  battery  control. 

With  this  system  of  control  there  is  not  the  resistance  loss  that 
is  present  in  the  resistance  control  system,  but  the  outfit  is  more 
costly. 

204.  Booster  Charge,  End  Cell  Discharge. — For  larger  self- 
contained  plants,  the  high  voltage  for  charging  is  obtained  by 
connecting  an  additional  generator  B,  called  a  booster,  so  that 
its  voltage  is  added  to  that  of  the  main  generators..  To  charge 
the  battery  in  Fig.  191,  the  switches  must  be  thrown  over  into 
position  Si  and  then,  if  the  generated  voltage  is  110  and  the  maxi- 
mum voltage  on  charge  is  160,  the  booster  has  to  supply  50  volts. 

The  booster  is  generally  driven  by  a  constant  speed  motor  and 


174      PRINCIPLES  OF  ELECTRICAL  ENGINEERING     [CHAP,  xxv 


is  excited  from  the  generator  mains,  the  field  rheostat  having 
sufficient  resistance  tov  allow  the  booster  voltage  to  be  reduced  to 
about  2  volts. 


FIG.  192. — Large  end  cell  switch. 

When  the  battery  is  fully  charged,  the  double  throw  switch 
is  thrown  over  into  the  position  shown  in  Fig.  191  so  as  to  connect 
the  battery  directly  across  the  mains,  the  discharge  voltage  is 


1800 


1600 


FIG.  193. — Daily  load  curve  on  a  small  station. 

then  regulated  by  the  end  cell  switch  C.  The  booster  may  be 
used  for  this  purpose  if  the  discharge  current  is  not  too  large,  see 
next  article. 


ART.  205]  OPERA  TION  OF  GENERA  TORS  AND  BA  TTERIES         1 75 

The  end  cell  switch  for  such  plants,  when  of  large  size,  gener- 
ally takes  the  form  shown  in  Fig.  192,  a  laminated  brush  being 
moved  across  a  series  of  contacts  by  a  motor-driven  operating 
screw.  This  motor  may  be  provided  with  push-button  control  if 
desired. 

205.  Capacity  of  Battery. — The  irregular  curve  in  Fig.  193  is 
the  load  curve  on  a  small  power  station,  and  a  battery  is  required 
to  supply  all  the  current  over  1000  amp. 

The  battery  discharge  =  cross  hatched  area  A 

=  1140  amp.  hours 
the  time  of  discharge  =  2. 7  hours 
the   average   discharge    current  =  420  amp. 
the  maximum  discharge  current  =  800  amp. 

When  the  discharge  takes  place  at  the  2.7-hour  rate,  the  capacity  of  the 
battery  is  only  75  per  cent,  of  the  normal  capacity,  see  page  153,  therefore 
the  normal  capacity  of  the  above  battery  =  1140/0.75 

=  1500  amp. -hours. 

To  recharge  the  battery,  about  20  per  cent,  more  ampere-hours  must  be 
put  in  than  were  taken  out  on  the  previous  discharge  when  this  discharge 
was  at  the  2.7-hour  rate,  see  page  154. 

therefore : 

the  charge  required  —  1140  X  1.20 

=  1370  amp.-hours 
-  shaded  area  B 

the  battery  charges  for  8.7  hours 
the  maximum  charging  current  =  230  amp. 

or  is  at  the  1500/230  =  6.5-hour  rate 
the  average  charging  current  =  1370/8.7  =  157  amp. 

the  booster  has  to  carry  a  maximum  of  230  amp.  and  must  be  designed  for 
a  maximum  voltage  of  50  the  output  is  therefore  50  X  230  =  11.5  kw. 

This  booster  could  not  be  used  to  help  the  battery  to  discharge 
because  the  800-amp.  discharge  current  would  burn  up  a  230- 
amp.  booster,  end  cells  must  therefore  be  supplied. 

In  working  out  this  problem  it  has  been  assumed  that  the  battery 
will  never  be  called  on  to  deliver  more  than  1140  amp.-hours  at 
the  2.7-hour  rate. 

No  attempt  has  been  made  to  determine  whether  or  not  the 
above  outfit  is  the  most  suitable  for  the  particular  service,  this 
is  a  question  which  can  be  answered  only  by  trial  of  different  com- 
binations of  generating  and  storage  equipment,  the  total  operat- 
ing cost  including  maintenance  and  depreciation  being  figured  out 
in  each  case. 


176      PRINCIPLES  OF  ELECTRICAL  ENGINEERING     [CHAP,  xxv 


206.  Batteries  for  Rapidly  Fluctuating  Loads. — The  load  on  the 
power  house  of  plants  such  as  rolling  mills  fluctuates  so  rapidly 
that   hand-operated  boosters  and  end  cells  cannot  follow  the 
fluctuations  and  automatic  control  becomes  necessary.     Only 
two  of  the  many  systems  that  have  been  devised  will  be  described, 
the  principle  in  each  case  being  to  control  the  field  excitation  of 
the  booster  by  means  of  the  load  current  in  such  a  way  that, 
when  the  load  is  heavy  the  booster  will  assist  the  battery  to 
discharge,  and  when  the  load  is  light  the  booster  will  assist  the 
generators  to  charge  the  battery. 

207.  The  Differential  Booster  in  its  simplest  form  is  connected 
as  shown  in  Fig.  194  and  is  supplied  with  a  set  of  shunt  coils  A 
and  a  set  of  series  coils  B  which  coils  are  connected  so  that  their 
m.m.fs.  are  in  opposition.     With  normal  load  on  the  generator, 

Series  Coils    Shunt  Coils 


r/'^X 


T 


Load 


FIG.  194. — Differential  booster  system  of  battery  control. 

the  m.m.f.  of  the  series  coils  is  equal  and  opposite  to  that  of  the 
shunt  coils  and  the  resultant  magnetic  field  is  zero,  so  that  the 
booster  voltage  is  zero  and  the  battery  neither  charges  nor 
discharges. 

If  the  load  on  the  generator  increases,  the  series  excitation  of 
the  booster  becomes  greater  than  the  shunt  excitation  and  the 
booster  voltage  is  then  added  to  the  battery  voltage  and  causes 
the  battery  to  discharge  and  carry  the  larger  part  of  the  excess 
load.  If  the  generator  load  now  becomes  less  than  normal  then 
the  series  excitation  decreases  and  the  shunt  excitation  is  now  the 
greater  so  that  the  booster  voltage  is  reversed,  opposes  the  battery 
voltage,  and  thereby  helps  the  generator  to  charge  the  battery. 

208.  Carbon  Pile  Regulator. — The  diagram  of  connections  for 
a  booster  system  controlled  by  a  carbon  pile  regulator  is  shown  in 
Fig.  195,  The  booster  B  is  excited  by  the  coil  E  which  takes 


ART.  208]  OPERATION  OF  GENERATORS  AND  BATTERIES         177 


current  from  a  small  exciter,  the  field  excitation  of  which  is  con- 
trolled by  a  regulator  consisting  of  two  carbon  piles  n  and  r2. 
These  carbon 'piles  are  subjected  to  pressure  by  the  lever  L,  to 


Series    ) 

/    r  Spring                    E      ] 

^-~^ 

Coil' 

Garbou 

Generator/^         \ 

Ecgulator 

-=-    Lou 
"  n      / 

r 

i 

Battery    — 

~=~L 

x. 

FIG.  195. — Carbon  pile  regulator  for  the  automatic  control  of  a  battery. 

one  end  of  which  is  attached  the  plunger  of  the  operating  so- 
lenoid S  and  to  the  other  end  a  spring  T,  the  tension  of  which  may 
be  adjusted  to  counterbalance  the  pull  of  the  solenoid  when  any 


FIG.  196. — Carbon  pile  regulator  for  a  battery. 

desired   current  is  flowing  through  it.     The  diagram  of  con- 
nections of  the  exciter  field  coil  circuit  is  shown  in  diagram  A. 
The  voltage  ei  tends  to  send  a  current  i\  =  e\/(r  +  n)  through 
12 


178      PRINCIPLES  OF  ELECTRICAL  ENGINEERING     [CHAP,  xxv 


the  coil  F  while  the  voltage  e2  tends  to  send  a  curreut  z'2  = 
e2/(V+r2)  through  the  same  coil  in  the  opposite  direction. 

If  the  generator  load  is  normal  then  the  pull  of  the  solenoid  S 
is  equal  to  the  pull  of  the  spring  T  and.ri  is  then  equal  to  r2,  i\  is 
equal  to  i2,  diagram  A,  and  no  current  flows  in  the  coil  F,  so  that 
the  exciter  voltage  and  the  booster  voltage  are  both  zero. 

If  the  generator  load  increases,  the  pull  on  r\  increases  and  its 
resistance  decreases  and  is  now  less  than  r2  so  that  i\  becomes 
greater  than  iz  and  current  flows  in  the  coil  F  in  the  direction  of 
i\  and  the  booster  voltage  adds  to  the  battery  voltage  and  causes 
the  battery  to  discharge  and  carry  the  larger  part  of  the  excess 
load. 


6000 


2000 


01234  56  789  10 

Minutes 

FIG.  197. — Load  curves  on  the  power  house  of  a  steel  mill. 


If  the  generator  load  now  becomes  less  that  normal,  the  pull  on 
TI  decreases  and  its  resistance  increases  and  is  now  greater  than 
r2  arid  current  now  flows  in  the  coil  F  in  the  direction  of  iZj  the 
booster  voltage  is  thereby  reversed  and  adds  to  the  generator 
voltage  and  thereby  helps  the  generators  to  recharge  the  battery. 

The  curves  in  Fig.  197  show  how  nearly  constant  the  generator 
load  may  be  maintained  by  such  a  regulator,  while  the  indicator 
cards  in  Fig.  198  show  the  effect  of  the  regulator  in  maintaining 
the  steam  consumption  constant,  an  operating  condition  which  is 
favorable  to  economy  in  steam  consumption. 


ART.  209]  OPERATION  OF  GENERATORS  AND  BATTERIES        179 

The  average  load  on  the  generator  is  that  at  which  the  pull  of 
the  solenoid  S  is  exactly  counterbalanced  by  the  tension  of  the 
spring  T,  for  then  the  booster  field  excitation  is  zero;  this  load 
may  be  adjusted  by  varying  the  tension  of  the  spring. 

One  advantage  of  the  externally  controlled  booster  over  the 
differential  booster  is  that  the  former  machine  is  a  standard  shunt 
generator  whereas  the  latter  has  special  series  field  coils  and 
heavy  cables  leading  to  these  coils;  some  idea  of  the  section  of 
copper  required  for  these  coils  and  cables  may  be  obtained  from 
Fig.  196  which  shows  the  section  of  copper  required  to  carry  the 
current  in  a  carbon  pile  regulator. 


A — Battery  in  circuit.  B — Battery  out  of  circuit. 

FIG.  198. — Indicator  cards  from  the  steam  engines  in  the  power  house  of  a 

steel  mill. 

209.  Floating  Batteries. — If  a  battery  is  connected  in  par- 
allel with  the  generator  and  with  the  load,  as  shown  in  Fig. 
199,  then  the  condition  to  be  fulfilled  before  the  battery 
will  carry  the  peak  load  is  that  the  voltage  regulation  of  the 
generator  shall  be  worse  than  that  of  the  battery,  so  that,  as  the 
load  increases,  the  generator  voltage  drops  below  that  of  the 
battery  and  the  battery  has  to  discharge,  while  if  the  load 
decreases,  the  generator  voltage  rises  above  that  of  the  battery 
and  the  battery  is  charged. 

Such  poor  regulation  is  not  desired  in  a  power  house,  but  is 
often  found  at  the  end  of  a  transmission  line.  If  the  length 
L  of  this  line  is  considerable  and  the  section  of  the  wire  is  small, 
then  a  heavy  load  on  the  line  will  cause  the  voltage  to  drop  suf- 
ficiently to  allow  the  battery  to  discharge,  while  with  a  light 
load  on  the  line  the  drop  is  small  and  the  voltage  is  then  high 
enough  to  recharge  the  battery. 

The  exact  equivalent  of  a  line  with  poor  regulation  is  shown 
in  Fig.  200,  which  represents  diagrammatically  a  hotel  power 
plant  supplying  lamps  L  and  elevator  and  other  motors  M. 


180      PRINCIPLES  OF  ELECTRICAL  ENGINEERING     [CHAP,  xxv 

The  generator  is  flat  compounded  to  maintain  the  voltage  con- 
stant across  the  lamps,  while  a  battery  is  inserted  to  carry  the 
peak  loads  produced  by  the  starting  of  the  elevators.  In  order 
that  the  battery  may  operate  properly,  there  must  be  a  con- 
siderable drop  in  voltage  between  the  generator  and  the  battery. 
This  is  arranged  for  by  placing  a  series  wound  booster  in  the 


Generator 


To  Load         Generator 
1 


To  Motors  M 


Booster 


To  Lighting 
Circuits  L 


FIG.  199. — Battery  floating  at  the 
end  of  a  line. 


FIG.  200. — Series  booster  system  of 
battery  control. 


circuit,  the  booster  being  driven  at  constant  speed  with  its 
voltage  opposing  that  of  the  generators,  so  that  if  the  elevator 
motors  take  a  large  current  the  strength  of  the  booster  series 
field  is  increased,  this  causes  the  booster  voltage  to  increase 
and  the  voltage  E^  to  drop  and  thereby  allows  the  battery  to 
discharge. 


CHAPTER  XXVI 
CAR  LIGHTING  AND  VARIABLE  SPEED  GENERATORS 

210.  The  essential  condition  to  be  satisfied  by  any  system 
of  electric  lighting  for  vehicles  is  that  the  voltage  across  the 
lamps  shall  be  approximately  constant  for  all  speeds  of  the 
vehicle  from  zero  up  to  the  maximum  value. 

For  train  lighting  on  steam  railroads  the  three  methods  at 
present  in  use  are: 

a.  Lighting  from  storage  batteries,  called  the  straight  storage 
system. 

b.  Lighting  from  a  constant  voltage  generator  placed  on  the 
locomotive  or  in  the  baggage  car,  called  the  head  and  end  system. 

c.  Lighting  from  generators  which  are  belted  to  the  car  axle. 
For  motor-car  lighting,  power  is  supplied  by  a  generator  which 

is  driven  by  the  engine. 

211.  Straight  Storage  for  Trains. — The  power  for  the  lighting 
load  in  this  case  is  supplied  entirely  by  storage  batteries  which 
are  carried  under  the  cars.     At  the  end  of  each  run  the  batteries 
must  be  recharged  or  else  replaced  by  fully  charged  batteries. 

212.  Head  and  End  System. — The  lighting  load  in  this  case 
is  supplied  by  a  100- volt  compound  wound  generator  driven  at 
constant  speed  by  a  turbine  which  takes  steam  from  the  loco- 
motive.    The  generating  unit  may  be  mounted  on  the  locomo- 
tive or  in  the  baggage  car. 

In  order  that  the  lights  on  the  train  may  not  go  out  if  the 
train  is  parted  or  if  the  locomotive  is  disconnected,  storage 
batteries  must  be  placed  on  at  least  the  front  and  the  rear  cars 
the  general  practice  being  to  place  a  battery  on  each  car.  The 
system  is  then  suitable  for  long  runs  where  cars  are  not  parted  and 
where  no  interchange  of  equipment  is  made  with  other  roads. 

The  standard  battery  equipment  for  such  service,  if  a  lead 
battery  is  used,  consists  of  32  cells  in  series  so  that 

the  battery  voltage  on  full  charge  =  32  X  2.65  =  85  volts 

at  the  beginning  of  discharge  =  32  X  2.2  =  70  volts 

at  the  end  of  discharge  =  32  X  1.8  =  57  volts 

181 


182     PRINCIPLES  OF  ELECTRICAL  ENGINEERING    [CHAP,  xxvi 

Since  60-volt  lamps  are  used,  an  automatic  regulator  must  be 
placed  in  the  lamp  circuit  of  each  car  so  as  to  maintain  the  lamp 
voltage  approximately  constant. 

213.  Carbon  Pile  Lamp  Regulator.-^-One  type  of  regulator 
which  depends  on  the  variable  resistance  of  a  carbon  pile  for  its 
operation  is  shown  on  the  middle  panel  of  Fig.  202  and  fs  also 
shown  diagrammatically  in  Fig.  201. 

If  the  battery  is  being  charged,  then  the  voltage  Eb  increases 
and  the  lamp  voltage  E\  also  increases  slightly.  This  increases 
the  pull  on  the  plunger  a  which  increases  the  pressure  on  the 
carbon  pile  r  and  decreases  its  resistance,  more  current  therefore 
flows  in  the  coil  b  which  is  connected  in  series  with  r  across  the 
lamp  circuit,  so  that  the  plunger  6  is  raised  and  the  pressure  on  the 
carbon  pile  R  is  decreased  and  its  resistance  increased,  the 


FIG.  201. — Lamp  circuit  regulator. 

greater  part  of  the  increased  battery  voltage  is  therefore  ab- 
sorbed by  the  resistance  R  and  the  lamp  voltage  remains  ap- 
proximately constant.  Due  to  the  multiplying  effect  of  the  aux- 
iliary solenoid  a  and  carbon  pile  r,  a  slight  change  in  the  voltage 
across  the  lamps  produces  a  considerable  change  in  the  pull  of  the 
solenoid  b. 

214.  The  Axle  Generator  Systems. — With  these  systems,  the 
lighting  equipment  of  each  car  is  self  contained  and  consists  of  a 
generator  driven  from  the  car  axle  and  a  storage  battery  to  supply 
power  when  the  car  is  at  standstill. 

An  automatic  switch  must  be  provided  so  that  the  generator 
shall  be  connected  to  the  battery  only  when  the  generator  voltage 
is  higher  than  the  battery  voltage  and  shall  be  disconnected 
when  its  voltage  is  lower.  The  generator  also  must  be  so  con- 
trolled that  the  charging  current  shall  not  be  excessive  and 


ART.  21G] 


CAR  LIGHTING 


183 


shall  moreover  be  reduced  automatically  as  soon  as  the  battery 
is  fully  charged.  The  combined  automatic  switch  and  generator 
regulator  shown  on  the  bottom  panel  of  Fig.  202  and  diagram- 
matically  in  Fig.  203  is  typical  of  the 
several  carbon  pile  regulating  systems 
on  the  market. 

215.  Automatic    Switch. — As   the  car 
speeds  up,  the  generator  voltage  increases 
and  when  it  reaches  a  value  which  is 
greater  than  the  maximum  battery  volt- 
age the  pull  on  the  plunger  P  due  to  the 
current  in  F  becomes  large  enough  to 
close  the  switch  S  and  connect  the  gen- 
erator and  the  battery  in  parallel.     The 
generator   then   delivers  current  to  the 
lamps  and  to  the  battery  which  current, 
flowing  through  the  coil  H,  adds  to  the 
pull  on  the  plunger  and  helps  to  keep 
the  switch  closed. 

If  the  car  now  slows  down,  the  speed 
finally  reaches  a  value  below  which  the 
generator  voltage  is  less  than  that  of 
the  battery,  and  current  flows  back 
through  the  coil  H  in  such  a  direction 
as  to  oppose  the  pull  of  F,  so  that  the 
switch  S  is  released  and  the  generator 
disconnected  from  the  circuit,  the  bat- 
tery being  left  connected  to  supply 
power  to  the  lamps. 

216.  Generator    Regulator. — As    the 
speed  of  the   generator    increases,    the 
voltage  of  the  generator  and  the  charging 
current   in   the   battery   both  increase. 
This  charging  current  is  limited  by  the 

regulator  shown  diagrammatically  in  Fig.  203  which  consists  of 
a  carbon  pile  rheostat  R  in  the  field  coil  circuit  and  a  solenoid 
B  in  the  battery  circuit.  The  weight  of  the  plunger  of  B  keeps 
the  carbon  pile  compressed,  while  the  upward  pull  of  the  sole- 
noid decreases  the  pressure  and  thereby  inserts  resistance  in  the 
shunt  coil  circuit  and  cuts  down  the  voltage  of  the  machine. 
The  regulator  is  adjusted  so  that  the  plunger  is  pulled  up  and 


FIG.  202. — Control  panel 
for  a  train  lighting  system. 


184     PRINCIPLES  OF  ELECTRICAL  ENGINEERING    [CHAP,  xxvi 

the  pressure  on  the  carbon  pile  is  relieved  as  soon  as  the  current 
reaches  the  safe  maximum  charging  rate  of  the  battery. 

If  this  were  the  only  provision  made  for  regulating  the 
generator,  then  the  battery  current  would  be  maintained  even 
after  the  battery  was  fully  charged,  this  would  cause  excessive 
gassing  of  the  battery  and  a  loss  of  water  from  the  electrolyte; 
the  generator  voltage  would  then  be  32  X  2.65  =  85  volts  with  a 
32-cell  battery. 

In  order  to  decrease  the  charging  current  once  the  battery  is 
fully  charged,  an  additional  relay  is  provided  which  limits  the 
generator  voltage  to  s,ome  value  lower  than  85  volts.  This  is 
accomplished  by  the  shunt  solenoid  A  which  is  connected  across 
the  generator  terminals  as  shown  in  Fig.  203.  The  weight  of 


FIG.  203. — Automatic  switch,  and  generator  regulator. 

plunger  of  this  solenoid  keeps  the  carbon  pile  compressed, 
but  when  the  generator  voltage  reaches  a  value  somewhat  lower 
than  85  volts  the  plunger  is  pulled  up  and  the  pressure  on  the 
carbon  pile  is  relieved  so  that  the  voltage  cannot  increase  further. 
217.  Pole  Changer. — The  polarity  of  the  leads  a  and  b,  Fig. 
203,  must  be  independent  of  the  direction  of  motion  of  the  car 
in  order  that  the  battery-charging  current  may  flow  in  the 
proper  direction,  and  the  generator  shunt  field  coils  are  connected 
across  ab  instead  of  across  the  generator  terminals  in  order  that 
the  magnetic  field  produced  may  always  be  in  the  same  direction 
as  that  due  to  residual  magnetism  and  may  therefore  build  up 
properly,  see  page  71.  If  then  the  direction  of  motion  of  the 
car  reverses,  the  polarity  of  the  generator  will  also  reverse,  so  that 


ART.  218]  CAR  LIGHTING  185 

provision  must  be  made  for  reversing  the  connections  between 
the  brushes  and  the  leads  a  and  b.  This  may  be  accomplished 
by  a  double  throw  switch  such  as  that  shown  at  T,  Fig.  203, 
which  is  thrown  over  by  a  mechanism  on  the  generator  shaft 
whenever  the  direction  of  rotation  of  the  generator  is  reversed. 

218.  The  Stone  Generator. — The  voltage  and  current  of  this 
generator  are  controlled  by  the  slipping  of  the  driving  belt.  The 
generator  is  suspended  by  an  adjustable  linkl/,  Fier.  204,  and  is 
therefore  free  to  swing  toward  or  away  from  the  driving  pulley 
on  the  axle.  The  belt  is  then  adjusted  to  pull  the  generator  out 
of  the  position  in  which  it  would  naturally  hang  and  the  tension 
put  on  the  belt  by  this  means  may  be  so  adjusted  by  the  linkL 
that  the  belt  will  slip  when  the  load  exceeds  a  certain  value. 

The  combined  automatic  switch  and  pole  changer  is  at  the 
commutator  end  of  the  machine,  see  Fig.  205.  The  contacts  B 


FIG.  204. — Suspension  of  a  slipping -belt  type  of  generator. 

are  fixed  while  the  contacts  A  are  carried  on  a  rocker  arm  C  which 
is  loose  on  the  shaft  and  is  carried  around  by  friction  in  the  direc- 
tion of  rotation  until  arrested  in  the  position  shown  by  a  stop, 
the  blade  AI  is  then  opposite  BI  and  A2  opposite  B2.  If  the  rota- 
tion had  been  in  the  opposite  direction,  the  arm  C  would  have 
been  carried  around  in  this  direction  until  A\  was  opposite  B2 
and  A 2  opposite  BI  when  the  motion  of  the  arm  would  have  been 
arrested  by  another  stop. 

When  the  speed  and  the  voltage  of  the  generator  reach  such  a 
value  that  the  generator  is  able  to  charge  the  battery,  then  the 
weights  w  are  thrown  out  and  the  arm  C  with  the1  switch  blades 
attached 'is  pushed  along  so  as  to  connect  the  generator  and  the 
battery  in  parallel.  If  the  speed  decreases  to  such  a  value  that 
the  generator  is  no  longer  able  to  charge  the  battery,  then  the 
spring  S  pulls  the  switch  blades  out  of  contact  and  disconnects 
the  generator. 

The  various  operations  take  place  in  the  following  order.     When 


186     PRINCIPLES  OF  ELECTRICAL  ENGINEERING    [CHAP,  xxvi 

the  car  starts  up,  the  rocker  arm  C  moves  around  to  its  proper 
position,  and  when  the  speed  becomes  high  enough,  the  switches 
AB  are  closed  and  the  generator  charges  the  battery.  As  the 
speed  increases,  the  generator  voltage  and  the  charging  current 
both  increase  until  the  load  becomes  large  enough  to  cause  the 
belt  to  slip,  the  speed  of  the  generator  will  then  increase  no 
further  nor  will  the  charging  current  in  the  battery  increase. 
This  system  has  given  satisfaction  although  its  efficiency  is  not 
so  high  as  that  of  some  of  the  other  systems  because  of  the  loss 
in  the  slipping  belt,  it  also  has  the  objectionable  feature  that  the 
charging  current  is  not  reduced  after  the  battery  is  fully  charged. 


a      if     IT 
FIG.  205. — The  Stone  train  lighting  generator. 


219.  Lighting  Generators  for  Motor  Cars. — The  equipment  for 
motor-car  lighting  is  similar  to  that  supplied  for  train  lighting 
except  that  a  pole  changer  is  not  required  since  the  generator, 
which  is  driven  by  the  engine,  always  rotates  in  one  direction 
when  the  speed  is  such  that  the  generator  is  connected  in  parallel 
with  the  battery. 

220.  Constant  Speed  Generators. — Several  machines  operate 
on  the  same  principle  as  the  Stone  train  lighting  generator,  the 
slipping  belt  being  replaced  by  a  slipping  clutch  in  order  to  make 
the  outfit  as  compact  as  possible.     This  clutch  slips  when  the 
generator  output  reaches  a  predermined  value. 

221.  Bucking  Field  Coils. — Differentially  compounded  genera- 
tors have  been  successfully  used  for  vehicle  lighting,    the  field 
windings  being  connected  as  shown  in  Fig.  206  where  A  is  a  shunt 
winding  which  is  connected  across  the  battery  and  therefore 


ART.  222] 


CAR  LIGHTING 


187 


gives  approximately  constant  excitation  while  B  is  a  series  wind- 
ing which  acts  in  opposition  to  or  bucks  the  shunt  winding. 

When  the  speed  of  the  generator  reaches  such  a  value  that 
the  generator  is  able  to  charge  the  battery,  the  automatic  switch 
closes.  As  the  speed  increases  further,  the  generator  voltage 
and  the  charging  current  both  increase,  but  this  charging  cur- 
rent passes  through  the  coils  B 
and  reduces  the  excitation  of  the 
machine,  so  that  the  charging 
current  is  limited.  The  charging 
current  can  never  exceed  the 
value  at  which  the  ampere-turns 
of  winding  B  is  equal  to  the  am- 
pere-turns of  winding  A  for  then 
the  flux  in  the  machine  and  the 
generated  voltage  would  both  be 
zero. 

222.  Vibrating  Contact  Regulator. — A  compact  and  efficient 
regulator  for  a  variable  speed  generator  is  shown  diagrammatically 
in  Fig.  207,  the  car  being  at  standstill.  As  the  car  speeds  up, 
the  generator  voltage  increases  and,  when  it  reaches  a  predeter- 
mined value,  the  pull  of  the  magnet  M  due  to  the  shunt  coil  a 
closes  the  main  switch  S  and  connects  the  battery  and  the  generator 


FIG.  206. — Generator  with  bucking 
field  coils. 


augpw 
li  I 


FIG.  207. — Vibrating  contact  regulator  for  a  variable  speed  generator. 

in  parallel.  The  generator  then  delivers  current  to  the  battery 
and  to  the  lamps  which  current,  flowing  through  the  coil  b,  adds 
to  the  pull  of  the  magnet  and  helps  to  keep  the  switch  S  closed. 
As  the  speed  of  the  generator  increases,  the  voltage  of  'the 
generator  and  the  charging  current  in  the  battery  both  increase. 
This  current  flows  through  the  coil  c  and,  when  it  becomes  equal 


188     PRINCIPLES  OF  ELECTRICAL  ENGINEERING    [CHAP,  xxvi 

to  the  safe  charging  current  of  the  battery,  the  pull  of  the  magnet 
N  closes  the  contact  T  and  short  circuits  the  generator  field  coils 
thereby  reducing  the  excitation  and  limiting  the  voltage  of  the 
machine,  the  battery  is  therefore  protected  from  excessive  charg- 
ing currents. 

In  order  to  decrease  the  charging  current  once  the  battery  is 
fully  charged,  an  additional  coil  d  is  placed  on  the  magnet  N  and 
is  connected  across  the  generator  terminals.  The  voltage  across 
coil  d  increases  as  the  battery  charges  until  finally  the  current  in 
coil  d  is  able  to  close  the  contact  T,  the  voltage  can  then  increase 
no  further  so  that  the  charging  current  gradually  decreases  and' 
is  comparatively  small  when  the  battery  is  fully  charged. 

For  motor-car  work,  a  lamp  circuit  regulator  is  considered  to 
be  an  unwarranted  complication  since  the  battery  voltage  changes 
slowly  and  the  lamp  voltage  may  be  adjusted  by  means  of  a 
hand  operated  rheostat  if  desired. 

223.  The  Rosenberg  Generator. — Diagram  A,  Fig.  208,  shows 
a  generator  with  the  field  coils  excited  from  a  battery  so  as  to 
produce  a  flux  4>.  When  the  armature  revolves  in  the  direction 
of  the  arrow,  e.m.fs.  are  induced  in  the  conductors,  the  directions 
of  which  are  shown  by  crosses  and  dots,  and  the  voltage  between 
the  brushes  BB  is  E1}  while  that  between  the  brushes  bb  is 
zero  since  the  e.m.fs.  in  the  conductors  from  61  to  a  are  opposed 
by  those  in  the  conductors  from  a  to  62.  If  the  brushes  BB  are 
joined,  then  current  will  flow  through  the  armature  and  will 
produce  an  armature  cross  field  0i0,  see  page  67. 

Consider  now  the  effect  of  this  cross  field  <f>la  acting  alone. 
Since  this  field  is  stationary  in  space  it  can  be  represented  by 
poles  NzSz  as  *n  diagram  B,  and  the  lines  of  force  $i0  are  cut 
by  the  armature  conductors  and  e.m.fs.  are  induced  in  the 
directions  shown.  The  voltage  between  the  brushes  bb  is  E2 
while  that  between  the  brushes  BB  is  zero.  If  the  brushes  bb 
are  joined,  then  current  will  flow  through  the  armature  and  will 
produce  an  armature  cross  field  <£2a. 

In  one  'form  of  the  actual  machine  shown  in  diagram  C,  the 
poles  NiSi  are  excited  from  the  battery  while  N2S2  have  no  field 
coils  but  carry  the  cross  flux  <t>ia.  The  brushes  BB  are  short 
circuited  while  the  brushes  bb  are  connected  to  the  load. 

As  the  speed  of  the  generator  increases  then,  referring  to  dia- 
gram A,  the  voltage  EI  increases  causing  the  current  /i  and  the 
flux  0i0  to  increase;  referring  now  to  diagram  B,  the  voltage 


ART.  223] 


CAR  LIGHTING 


189 


E%  increases  with  the  flux  <£ia  and  causes  /2  and  $20  to  increase. 
But  02a  opposes  0  and  demagnetizes  the  machine,  and  the 
greater  the  speed  the  greater  the  demagnetizing  effect,  so  that 
/2  increases  by  a  smaller  and  smaller  amount  and  over  a  wide 
range  of  speed  remains  practically  constant.  The  current  72 
can  never  exceed  the  value  with  which  <£2a  is  equal  to  4>  because 
then  the  flux  in  the  machine  would  be  zero. 

Such  a  machine  is  therefore  a  constant  current  generator, 


C  Complete  Machine. 

FIG.  208. — Rosenberg  train  lighting  generator. 

and  by  adjusting  the  exciting  current  //,  the  current  72  can  be 
limited  to  the  normal  charging  current  of  the  battery;  the  charg- 
ing current  however  does  not  decrease  as  the  battery  becomes 
fully  charged,  a  disadvantage  this  generator  possesses  in  common 
with  the  slipping  belt  type  of  generator. 

The  operation  of  this  generator  is  described  in  detail  because 
it  has  several  interesting  applications.  It  is  used  for  train  lighting, 
when  it  is  called  the  Rosenberg  generator;  a  similar  machine  called 
the  C.  A.  V.  generator  is  used  for  motor  cars.  The  machine  has 


190     PRINCIPLES  OF  ELECTRICAL  ENGINEERING    [CHAP,  xxvi 

also  been  used  as  a  constant  current  generator  for  the  operation  of 
the  arc  of  large  searchlights  and  also  for  arc  welding,  since  it  is 
practically  fool-proof  and  can  be  short  circuited  across  the  operat- 
ing brushes  bb  without  the  current  exceeding  the  normal  value. 


CHAPTER  XXVII 
ALTERNATING  VOLTAGES  AND  CURRENTS 

224.  The  Simple  Alternator.— If  the  coil  abed,  Fig.  209,  be 
rotated  between  the  poles  N  and  S  so  that  the  conductors  ab 
and  cd  cut  lines  of  force,  an  alternating  e.m.f.  will  be  found  be- 
tween/ and  g,  the  ends  of  the  coil.  The  direction  of  the  e.m.f.  in 
each  conductor,  found  by  the  right-hand  rule  (page  9),  is  shown 
in  diagrams  A,  B,  C  and  D  for  different  positions  of  the  conductors 
relative  to  the  poles.  In  diagram  A  the  conductors  are  not  cut- 


FIG.  209. — Simple  two-pole  alternator. 

ting  lines  of  force,  and  the  e.m.f.  between  /  and  g  is  zero.  In 
diagram  B  the  e.m.fs.  in  the  conductors  are  in  such  a  direction 
as  to  force  current  from/  to  g  through  the  external  circuit,  there- 
fore /  is  the  positive  and  g  the  negative  terminal  of  the  machine. 
In' diagram  C  the  e.m.f.  between  /  and  g  is  again  zero.  In  dia- 

191 


192    PRINCIPLES  OF  ELECTRICAL  ENGINEERING    [CHAP,  xxvn 

gram  D  the  e.m.fs.  in  the  conductors  are  in  such  a  direction  as  to 
force  the  current  from  g  to  /  through  the  external  circuit,  therefore 
g  is  now  the  positive  and  /  the  negative  terminal.  The  current  in 
the  external  circuit  connecting  /  and  g  therefore  alternates;  that 
is,  electricity  oscillates  backward  and  forward  in  the  circuit. 

If  the  e.m.f.  between  /  and  g  is  plotted  against  time  then  a 
curve  such  as  that  in  Fig.  209  is  obtained. 


FIG.  210. — Two-pole  revolving-field  alternator. 


Alternators  are  generally  made  with  stationary  conductors 
and  a  revolving  field,  as  shown  diagrammatically  in  Fig.  210. 
The  direction  of  the  e.m.f.  in  each  conductor,  shown  at  different 
instants  in  diagrams  A,  B,  C  and  D,  may  be  found  by  the  right- 
hand  rule;1  it  must  be  noted  that  in  the  case  of  a  revolving-field 
machine  the  thumb  is  pointed  in  a  direction  opposite  to  that  of 
the  motion  of  the  poles  since,  according  to  the  rule,  it  must  be 

1  Thumb — direction  of  motion  of  conductor  relative  to  magnetic  field. 
Forefinger — direction  of  lines  of  force. 
Middle  finger — direction  of  e.m.f. 


ART.  226]  ALTERNATING  VOLTAGES  AND  CURRENTS 


193 


pointed  in  the  direction  of  motion  of  the  conductors  relative  to 
the  poles. 

225.  The  Wave  Form. — If  the  air-gap  clearance  under  the  pole 
is  uniform  in  thickness  then  the  lines  of  force  crossing  the  air 
gap  are  spaced  as  shown  in  Fig.  211,  and  the  e.m.f.  in  a  conductor, 
being  proportional  to  the  rate  of  cutting  of  lines  of  force,  varies 
as  in  curve  A.  By  shaping  the  pole  face,  however,  as  in  Fig.  212, 
the  flux  density  in  the  air  gap  and  therefore  the  rate  of  cutting  of 
lines  of  force  may  be  so  regulated  that  the  e.m.f.  in  a  conductor 
shall  vary  according  to  a  sine  law  as  shown  in  curve  B.  The  e.m.f. 
is  then  said  to  be  simple  harmonic  and  may  be  represented  by  the 
formula  e  =  Em  sin  0. 


FIG.  211.  FIG.  212. 

FIGS.  211  AND  212. — Wave  form  of  electromotive  force. 

226.  The  Oscillograph.— The  shape  or  form  of  the  e.m.f.  wave 
of  an  alternator  may  readily  be  determined  by  means  of  an  instru- 
ment called  an  oscillograph,  the  essential  parts  of  which  are  shown 
in  Fig.  213. 

In  the  narrow  gap  between  the  poles  NS  of  a  magnet  are 
stretched  two  parallel  conductors  ss  formed  by  bending  a  strip 
of  phosphor  bronze  back  on  itself  over  an  ivory  pulley  P.  A 
spiral  spring  attached  to  this  pulley  serves  to  keep  a  uniform 
tension  on  the  strips,  and  a  guide  piece  L  limits  the  length  of  the 
vibrating  portion  to  the  part  actually  in  the  magnetic  field.  A 
small  mirror  M  bridges  across  the  strips  as  shown. 

If  current  is  passed  through  the  strips  ss  then  one  strip  will 

13 


194    PRINCIPLES  OF  ELECTRICAL  ENGINEERING    [CHAP,  xxvn 


advance  and  the  other  will  recede  and  the  mirror  will  thereby 
be  tilted  about  a  vertical  axis.  If  the  current  is  alternating  then 
the  mirror  will  tilt  backward  and  forward  with  a  frequency  equal 
to  that  of  the  current,  and  the  deflection  will  be  proportional  to 
the  current.  (The  natural  frequency  of  vibration  of  the  mirror 
is  at  least  fifty  times  the  frequency  of  the  current.) 

If  now  a  beam  of  light  is  directed  on  the  mirror,  the  reflected 
beam  will  move  to  and  fro  in  the  horizontal  plane,  its  displace- 


M 


FIG.  213. — The  oscillograph. 


FIG.  214. — Six-pole  alternator. 

ment  from  the  zero  position  c  being  proportional  to  the  current 
flowing,  so  that  if  a  photographic  film/  is  moved  downward  at  a 
constant  speed  a  curve  will  be  traced  on  it  by  the  beam  of  light, 
which  curve  will  be  the  wave  of  the  e.m.f.  applied  at  the  oscillo- 
graph terminals. 

227.  Frequency. — In  the  two  pole  machine  shown  in  Fig.  209, 
the  e.m.f.  between  the  terminals  passes  through  a  complete  cycle 
while  the  machine  makes  one  revolution.  In  the  six-pole  machine 


ART.  228]  ALTERNATING   VOLTAGES  AND  CURRENTS  195 

shown  in  Fig.  214,  the  e.m.f.  in  any  conductor  a  passes  through 
three  cycles,  one  cycle  per  pair  of  poles,  while  the  machine  makes 
one  revolution. 

If,  in  an  alternator,  p  is  the  number  of  poles  then 

the  cycles  per  revolution    =    9 

and  the  cycles  per  sec.,  called  the  frequency,  =  ^  X  ~^r~ 

_  P  X  r.p.m. 


and  is  represented  by  the  symbol  /. 

The  frequencies  generally  found  in  practice  in  America  are 
25  and  60  cycles  per  sec.,  while  in  Europe  25  and  50  cycles  per 
sec.  are  more  common. 

A  60-cycle  alternator  has  24  poles,  at  what  r.p.m.  must  it  be  run? 

/  =  p  X  r.p.m/120 
therefore  60  =  24  X  r.p.m/120 
and    r.p.m    =  300. 

The  following  table  gives  the  relation  between  poles,  speed  and 
frequency: 


Revolutions  per  minute 

Poles 

25  cycles 

50  cycles 

60  cycles 

2 

1500 

3000 

3600 

4 

750 

1500 

1800 

6 

.      500 

1000 

1200 

8 

375 

750 

900 

p             !            3000/7? 

6000/p 

7200/p 

It  is  important  to  note  that  an  alternator  has  a  definite  speed 
for  a  given  frequency  and  cannot  be  run  above  or  below  that 
speed  without  changing  the  frequency.  In  a  direct- current 
generator,  the  voltage  may  be  varied  by  varying  the  speed,  but 
in  the  case  of  an  alternator  this  cannot  be  done  without  at  the 
same  time  changing  the  frequency. 

228.  Vibrating  Reed  Type  of  Frequency  Meter. — In  this  type 
of  instrument  a  number  of  steel  strips  are  fastened  at  one  end  as 
shown  in  Fig.  215,  while  the  current  whose  frequency  is  to  be 
determined  is  passed  through  the  coil  A .  The  reeds  are  attracted 
twice  in  a  cycle  by  the  electromagnet  B  and  that  reed  which  has 
a  natural  frequency  equal  to  twice  the  frequency  of  the  current 


196    PRINCIPLES  OF  ELECTRICAL  ENGINEERING    [CHAP,  xxvii 


Tibrating  Strips 


A — Interior  construction. 


i  ii  ii  mi  mi  mm  n 

85     90     95    100   105 


B — Scale  when  the  frequency  is  100  cycles.        C — External  appearance. 
FIG.  215. — Vibrating  reed  type  of  frequency  meter. 


FIG.  216. — Average    value    of    an    alternating    electromotive    force 


ART.  230]  ALTERNATING   VOLTAGES  AND  CURRENTS 


197 


will  be  sot  in  violent  vibration.  The  reeds  have  their  free  ends 
whitened  and  appear  as  white  bands  when  vibrating.  The  ex- 
ternal appearance  of  such  an  instrument  is  shown  in  diagram  C. 

229.  Average  Value  of  Current  and  Voltage. — The  average 
value  of  an  alternating  current  or  voltage  is  zero  because  similar 
sets  of  positive  and  negative  values  occur.     The  term  average 
is  generally  applied  to  the  average  value  during  the  positive  part 
of  a  cycle  as  indicated  in  Fig.  216. 

2 

Eav,  the  average  e.m.f.  =     Eml 

7T 

2 
lav,  the  average  current  =     Im 

7T 

230.  The   Heating  Effect   of  an  Alternating   Current.— If  a 
direct  current  /  is  forced  through  a  resistance  of  R  ohms  then  the 
power  transformed  into  heat  =  PR  watts. 


FIG.  217. 


If  an  alternating  current  i  =  Im  sin  6  is  forced  through  the 
same  resistance,  then  the  power  transformed  into  heat  at  any  in- 


X    7T     = 


Emsm  e  de 


=  -     Emcos  e 


therefore 


198    PRINCIPLES  OF  ELECTRICAL  ENGINEERING    [CHAP,  xxvn 

stant  =  i2R  watts  and  the  average  power  transformed  into  heat 

=  the  average  value  of  i2R,  see  Fig.  217. 
=  the  average  value  of  (Im  sin  0)2  R 
=  Im2  R  (average  value  of  sin2  6)  } 


where  7e//  =  JW/V2  is  called  the  effective  current. 

When  an  alternating  current  or  voltage  is  specified  it  is  always 
the  effective  value  that  is  meant  unless  there  is  a  definite  state- 
ment to  the  contrary,  thus  an  alternating  current  of  10  amp.  is 
one  which  has  the  same  heating  effect  as  10  amp.  direct  cur- 

rent and  has  a  maximum  value  of  10  V2  or  14.1  amp.  and  an 

2 
average  value  of      X  14.1  or  9  amp. 

231.  Symbols.  —  Hereafter  in  the  text  the  following  symbols 
shall  be  used  for  alternating  voltages  and  currents: 

e  =  Em  sin  6;  i      =/wlsin0;  the  instantaneous  value 
Em  Im  ;  the  maximum  value 

2  2 

Eav  =   ~  Em  Iav   =    -  Im   ',  the  average  value 

7T  7T 

ET  T 

E    =  77-      7      =    ~j~     ;  the  effective  value 

232.  Voltmeters  and  Ammeters  for  Alternating  -current  Cir- 
cuits. —  The  moving  coil  permanent  magnet  type  of  instrument 
as   used  for   direct-current   circuits   was   described   on  page  8. 
If  this  type  of  meter  was  connected  into  an  alternating-  current 
circuit  the  moving  coil  would  be  acted  on  by  forces  tending  to 
turn  it  first  in  one  direction  and  then  in  the  other  but,  due  to 
its  inertia,  the  coil  itself  would  not  move  and  the  reading  would 
be  zero. 

/> 

!The  average  value  of  sin2  6  X  IT  =  I     sin20  dd 

r 

=  -  I   J  (cos  26  -  1)  de 
=  -  U  \  sin  2(9  -  0) 

*  o 

7T 

=  2 
thereforeHhe  average  value  of  sin2  8  =  1/2. 


ART.  232]  ALTERNATING  VOLTAGES  AND  CURRENTS 


199 


In  order  that  a  moving  coil  instrument  may  be  used  for  the 
measurement  of  alternating  currents,  the  magnetic  field  and  the 
current  in  the  moving  coil  must  alternate  together.  This  result 
is  obtained  by  replacing  the  permanent  magnet  by  an  electro- 
magnet as  shown  in  Fig.  218.  The  current  to  be  measured  is 
passed  through  the  stationary  coils  A  and  through  the  moving 
coil  C  in  series,  and  the  sides  of  the  moving  coil  are  then  acted  on 
by  forces  which  turn  the  coil  against  the  tension  of  the  spring  S. 


FIG.  218. — Electrodynamometer  type  of  instrument. 

These  forces  are  proportional  to  the  current  i  in  the  coil  C 
and  to  the  flux  <£  produced  by  the  current  i  in  the  coils  A ;  the 
forces  are  therefore  proportional  to  i*  and  the  average  turning 
force  on  the  coil  C  while  the  current  alternates  is  proportional  to 
the  average  value  of  i2  or  to  the  effective  current. 

Such  an  instrument  may  be  used  to  measure  both  direct  and 
alternating  currents  and  the  reading  would  be  the  same  for  10 
amp.  direct  current  as  for  10  amp.  effective  alternating  current. 


CHAPTER  XXVIII 

REPRESENTATION  OF  ALTERNATING  CURRENTS  AND 

VOLTAGES 

233.  Part  of  a  rotating  field  alternator  is  shown  diagram- 
matically  in  Fig.  219.     As  the  field  rotates,  stationary  conductors 


TIG.  219. 


TIG.  220. 


such  as  a  are  cut  by  lines  of  force  and  the  e.m.f.  in  these  con- 
ductors varies  as  shown  in  Fig.  220  and  goes  through  one  cycle 
for  every  pair  of  poles  that  pass. 


;>,TT 


A  B 

FIG.  221. — Representation  of  an  alternating  voltage. 

234.  Electrical  Degrees. — If  the  vector  op  rotate  in  the  coun- 
ter-clock direction  and  the  angle  8  is  measured  from  the  z-axis 
then  om,  the  projection  of  op  on  the  y-axis  =  op  sin  6  and  its  value, 
plotted  in  diagram  B,  passes  through  one  cycle  while  B  changes 
through  360  degrees.  If  now  op  is  drawn  to  scale  equal  to 

200 


ART.  235]  ALTERNATING  CURRENTS  AND   VOLTAGES  201 

Em,  Fig.  220,  then,  oni  =  Em  sin  6  and  therefore  represents  the 
instantaneous  e.m.f.  e,  the  curves  in  Figs.  220  and  221  will 
therefore  be  alike  in  every  respect  if  the  angle  the  machine 
moves  through  in  generating  one  cycle  of  e.m.f.  is  called  360 
electrical  degrees;  in  the  machine  in  Fig.  219  this  angle  is  that 
between  two  consecutive  like  poles.  From  this  it  follows  that 
a  curve  such  as  that  in  Fig.  221  may  be  used  to  represent  the 
voltage  generated  by  an  alternator  with  any  number  of  poles  and 
is  the  curve  that  would  be  obtained  by  an  oscillograph. 

235.  Vector  Representation  of  Alternating  Voltages  and  Cur- 
rents.— It  is  generally  assumed  that  these  quantities  vary  accord- 


FIG.  222. — Representation  of  an  alternating  voltage  and  current. 

ing  to  a  sine  law  and  can  therefore  be  represented  by  sine  curves 
as  shown  in  Fig.  222,  where 

i,  the  current  at  any  instant  =  Im  sin  0 
e,  the  voltage  at  any  instant  =  Em  sin  0' 

For  much  of  the  work  on  alternating-current  circuits  and 
machines  it  is  more  convenient  to  represent  alternating  voltages 
and  currents  by  the  corresponding  vectors  /  and  E,  Fig.  222,  from 
which  vectors  the  sine  curves  may  be  obtained  when  desired  by 
plotting  the  vertical  components  i  and  e  against  the  angle  0,  the 
vectors  being  rotated  in  the  counter-clock  direction. 

If  two  oscillographs  are  used  as  in  Fig.  223  one  of  which,  A., 
gives  the  voltage  curve  while  the  other,  B,  gives  the  current  curve, 
it  will  be  found  that  the  current  and  voltage  do  not  necessarily 
reach  their  maximum  values  at  the  same  instant  but  that  curves 
such  as  those  in  diagrams  A,  B  and  C,  Fig.  223,  may  be  obtained, 
depending  on  the  kind  of  load  connected  to  the  circuit.  The 
reasons  for  the  displacement  of  the  current  relative  to  the  voltage 
are  taken  up  in  Chapters  29  and  30;  it  is  necessary  however  to 
take  up  at  this  point  the  method  of  representing  such  curves  by 
vectors. 


202    PRINCIPLES  OF  ELECTRICAL  ENGINEERING  [CHAP,  xxvm 

If  two  vectors  such  as  E  and  /,  Fig.  224.  be  rotated  in  the 
counter-clock  direction  at  the  same  rate,  the  ?,ngle  a  between  them 
will  remain  unchanged  and  the  vertical  components  e  =  Em  sin  6 
and  i  =  Im  sin  6,  which  are  shown  at  three  instants  a,  b  and  c, 


f 


A  Current  Lags  Voltage  by 
by  tt  Degrees 


B    Current  in  Phase 
with  Voltage 


-J4- 


C    Current  Leads  Voltage 
by  a  Degrees 

FIG.  223. — Phase  relation  between  current  and  voltage. 

when  plotted  against  the  angle  through  which  the  vectors  have 
moved  measured  from  any  base  line  ox,  will  give  the  sine  curves 
of  E  and  7  which  curves  represent  a  voltage  and  a  current  of  the 
same  frequency,  the  voltage  E  reaching  its  maximum  value  a 
degrees  before  the  current  becomes  a  maximum. 


ART.  236]  ALTERNATING  CURRENTS  AND   VOLTAGES 


203 


When  the  current  and  voltage  reach  their  maximum  values  at 
the  same  instant  they  are  said  to  be  in  phase  with  one  another. 
When  they  reach  their  maximum  values  at  different  instants 
they  are  out  of  phase  and  the  current  is  said  to  be  leading  or  lag- 


FIG.  224. — Representation  of  a  lagging  current. 


FIG.  227. — The  sum  of  two  alternating  voltages  of  the  same  frequency. 

ging  according  as  it  becomes  a  maximum  before  or  after  the  vol- 
tage has  reached  its  maximum  value. 

236.  The  Sum  of  Two  Alternating  Voltages  of  the  Same 
Frequency. — If  two  direct- current  generators  are  connected  in 


204    PRINCIPLES  OF  ELECTRICAL  ENGINEERING  [CHAP,  xxvm 

series  as  in  Fig.  225,  the  resultant  voltage  E*  is  the  numerical 
sum  of  EI  and  E2  the  voltages  of  the  two  machines. 

If  two  alternators  are  connected  in  series  as  in  Fig.  226,  the 
voltage  es  at  any  instant  is  the  numerical  sum  of  e\  and  e2  the 
voltages  of  the  two  machines  at  that  instant.  In  the  particular 
case  shown,  the  voltage  of  the  second  machine  lags,  or  reaches 
its  maximum  value  later  than  that  of  the  first  machine  by  the 
time  required  for  the  poles  to  pass  through  a  degrees,  and  the 
curves  representing  the  voltages  of  the  two  machines  are  e\  and 
e2,  Fig.  227.  The  points  on  the  resultant  curve  63  are  obtained 
by  adding  together  the  values  of  e\  and  e2  at  different  instants. 

The  voltages  of  the  two  machines  may  be  represented  by 
the  vectors  EI  and  E2  drawn  to  scale  with  an  angle  a  between 
them  because,  if  these  two  vectors  are  rotated  together  in  the 
counterclock  direction  with  this  fixed  angle  a  between  them,  then 
the  vertical  components  e\  and  e2,  when  plotted  against  the 
angle  turned  through  by  the  vectors,  will  give  the  curves  e\ 
and  e2  in  their  proper  phase  relation. 

The  resultant  of  these  two  electromotive  forces  is  the  vector 
sum  obtained  by  the  parallelogram  law  and  is  Es  which  repre- 
sents the  resultant  voltage  both  in  magnitude  and  in  phase 
relation. 


CHAPTER  XXIX 
INDUCTIVE  CIRCUITS 

237.  Inductance. — It  has  been  shown  that  whenever  there  is  a 
change  in  the  current  flowing  through  a  circuit,  an  e.m.f.  of  self 
induction  is  induced  which  opposes  the  change  of  the  current, 
see  page  11. 

In  Fig.  228,  when  the  switch  k  is  closed  a  current  begins  to 
flow  in  the  coil  and  as  this  current  increases  in  value  the  flux.  < 


Time 


FIG.  228. — Growth  and  decay  of  current  in  an  inductive  circuit. 

threading  the  coil  also  increases.  Due  to  the  change  in  the  flux, 
an  e.m.f.  of  self  induction  is  induced  in  the  coil  which,  accord- 
ing to  Lenz's  law,  page  10,  acts  in  such  a  direction  as  to  oppose 
the  increase  of  the  current. 

If,  after  the  current  has  reached  its  final  value,  the  switch  k 
is  suddenly  opened,  the  current  in  the  coil  decreases,  the  flux 
threading  the  coil  also  decreases  and  causes  an  e.m.f.  of  self 
induction  to  be  induced  in  such  a  direction  as  to  oppose  the  de- 
crease of  the  current.  This  e.m.f.  is  generally  large  enough  to 
maintain  the  current  between  the  switch  contacts  for  a  short 

205 


206     PRINCIPLES  OF  ELECTRICAL  ENGINEERING    [CHAP,  xxix 

interval  as  they  are  opened  and  accounts  for  much  of  the  flash- 
ing that  is  seen  when  a  switch  is  opened  in  a  circuit  carrying  cur- 
rent. The  growth  and  decay  of  current  in  such  a  circuit  is  shown 
by  the  curves  in  Fig.  228. 

That  property  of  an  electric  circuit  whereby  it  opposes  a  change 
in  the  current  flowing  is  called  the  self  induction  or  the  inductance 
of  the  circuit;  the  two  terms  have  the  same  meaning  but  the 
term  inductance  is  generally  used  in  engineering  work. 

238.  Make  and  Break  Spark  Ignition. — One  method  of  igniting 
the  gas  in  a  gas  engine  cylinder  is  based  on  the  above  properties  of 
the  inductive  circuit;  the  essential  parts  of  the  mechanism  are 
shown  in  Fig.  229. 


Cylinder 
X 

ro     rr~~i 

^ 

K 

I 

1 

\^ 

•Insulation 


End  view. 


Section  through  cylinder. 


FIG.  229. — Make  and  break  method  of  gas  ignition. 

When  the  contact  at  x  is  closed,  current  flows  in  the  direction 
of  the  arrow.  When  the  cam  c  has  reached  a  predetermined 
position,  the  spring  s  opens  the  contact  x  and  the  current  is  main- 
tained across  the  gap  by  the  inductance  coil  L. 

The  current  in  the  circuit  changes  as  shown  in  Fig.  228  so  that 
the  contact  x  must  be  closed  long  enough  to  allow  the  current  to 
grow  to  its  full  value,  but  should  not  be  closed  too  long  or  the 
batteries  will  run  down. 

239.  The  Coefficient  of  Self  Induction  of  a  circuit  is  defined 
as  the  flux  interlinkages  per  unit  current.  If,  in  Fig.  228,  a 
current  /  produces  a  flux  </>  which  links  the  n  turns  of  the  coil 
then  the  flux  interlinkages  =  n<f>  and  the  flux  interlinkages  per 


unit  current  =  -j-m 


ART.  240] 


INDUCTIVE  CIRCUITS 


207 


A  larger  unit  is  used  in  practice  and  L,  the  coefficient  of  self 
induction  in  henries  =  -j-10~8  where  /  is  in  amperes. 

If  the  current  /  is  changing,  the  flux  0  is  also  changing  and  a 
voltage  of  self  induction  is  induced  in  the  coil  which  is  equal  to 

esi  =  —  n-7rlO~8  volts,  see  page  91 


and  since  L  =  -10~ 


therefore  Ldi  = 


10~8 


and  esi  =  —  L  ,  ,  volts 

240.  Alternating  Currents  in  Inductive  Circuits.  —  If  an  al- 
ternating current  is  flowing  in  an  inductive  circuit  then,  since  the 
current  is  always  changing,  there  must  be  an  induced  e.m.f.  of 
self  induction  opposing  the  change.  If  the  current  is  represented 


•/MI 


Eu, 


FIG.  230. — Voltage  and  current  relations  in  an  inductive  circuit, 
by  curve  /,  Fig.  230,  then  between  a  and  6,  during  which  interval 
of  time  the  current  is  decreasing,  the  eim.f.  of  self  induction,  to 
oppose  this  decrease,  must  be  positive,  while  between  b  and  c,  dur- 
ing which  interval  of  time  the  current  is  increasing,  the  e.m.f.  of 
self  induction,  to  oppose  this  increase,  must  be  negative.  At  the 
instants  a,  b  and  c  the  current  is  not  changing  and  at  these  in- 
stants the  e.m.f.  of  self  induction  must  be  zero.  The  e.m.f.  which 
satisfies  all  those  conditions  is  represented  by  the  curve  Eti,\ 
Fig.  230. 

In  order  to  force  an  alternating  current  /  through  a  circuit,  the 
applied  e.m.f.  must  be  large  enough  to  overcome  the  e.m.f.  of 
self  induction  and  also  the  resistance  of  the  circuit  and,  in  the 

'The  minus  sign  is  used  because  the  e.m.f.  opposes  the  change  of  the 
flux. 

2It  is  important  to  note  that  the  generated  voltage  E,i  lags  the  current 
i  and  therefore  the  flux  B  by  90  degrees. 


208      PRINCIPLES  OF  ELECTRICAL  ENGINEERING    [CHAP,  xxix 

extreme  case  of  an  inductive  circuit  of  negligible  resistance,  the 
applied  e.m.f.  must  be  equal  and  opposite  to  the  e.m.f.  of  self 
induction.  The  applied  e.m.f.  in  this  latter  case  is  represented  by 
the  curve  Ea,  Fig.  230,  and  it  is  important  to  note  that,  in  the 
case  of  an  inductive  circuit  of  negligible  resistance,  the  current  I 
lags  the  applied  voltage  Ea  by  90  degrees. 

241.  Voltage  and  Current  Relations.—  The  current  /  in  Fig.  230 
changes   from  —  Im  to  +  Im  in  the  time  of   half  a  cycle  or  in 

n-*  seconds  so  that 
4/ 

Eav,  the  average  voltage  of  self  induction  between  b  and  c, 

=  L  (average  value  of  -*; 

k.\ 

=  L      1      =4fLIm 

\2// 
and  Emax,  the  maximum  voltage  of  self  induction 


=    2  TT/LIm 

therefore  Eeff  =  2  7r/L/e// 

In  direct  current  circuits,  E  =  IR.  In  inductive  circuits  of 
negligible  resistance,  E  =  IX  where  X,  called  the  inductive 
reactance,  is  expressed  in  ohms  and  is  numerically  equal  to 


An  alternating  e.m.f.  of  110  volts  sends  2.2  amperes  through  an  inductance 
coil  of  negligible  resistance  at  60  cycles.     Find  the  reactance  at  60  cycles 
and  find  also  the  coefficient  of  self  induction. 
X,   the  reactance  =  E/I 

=  110/2.2  =  50  ohms 

X 

L,  the  coefficient  of  self  induction  =       H—  - 

^0 
-      2  ,T60     =0-133  henry 

If  the  voltage  applied  to  the  above  .circuit  is  kept  constant  at  110,  find 
the  current  that  will  flow  through  the  inductance  at  30,  60,  90  and  120  cycles 

X,  the  reactance  =  2irfL  =  50  ohms  at  60  cycles,  from  last  problem 
=  25  ohms  at  30  cycles 
=  75  ohms  at  90  cycles 
=  100  ohms  at  120  cycles 

/,  the  current  =  E/X  =  110/25     =  4.4  amp.  at  30  cycles 
=  110/50     =  2.2  amp.  at  60  cycles 
=  110/75     =  1.47  amp.  at  90  cycles 
=  110/100  =  1.1  amp.  at  120  cycles 


ART.  243]  INDUCTIVE  CIRCUITS  209 

The  current  is  inversely  proportional  to  the  frequency  because,  the  greater 
the  frequency  the  smaller  the  current  required  to  give  the  same  voltage  of 
self  induction. 

242.  Power  in  an  Inductive  Circuit. — The  power  in  a  circuit 
at  any  instant  in  watts  is  the  product  of  e  and  i  the  voltage  and 
current  at  that  instant.  In  an  inductive  circuit  of  negligible 
resistance  the  current  lags  the  applied  voltage  by  90  degrees  and 
the  curves  representing  e  and  i  are  shown  by  light  lines  in 
Fig.  231. 

At  the  instants  a  and  b  the  voltage  is  zero  so  that  the  power 
is  zero  at  these  instants;  it  is  also  zero  at  instants  g,  d  and  / 


FIG.  231. — Voltage,  current  and  power  in  an  inductive  circuit. 

when  the  current  is  zero.  Between  g  and  a  the  voltage  and  cur- 
rent are  in  the  same  direction  so  that  power  is  positive  or  energy 
is  being  put  into  the  circuit,  while  between  a  and  d  the  current 
and  voltage  are  in  opposite  directions  so  that  power  is  negative 
or  energy  is  being  taken  from  the  circuit;  the  average  power 
in  the  circuit  is  zero. 

A  hypothetical  mechanical  circuit  with  somewhat  similar 
properties  is  shown  in  Fig.  232.  As  the  weight  W  falls,  work  is 
done  on  the  flywheel  and  the  velocity  increases  until  the  rope  is 
all  unwound;  the  flywheel  continues  to  rotate  and  now  raises 
the  weight,  so  that  work  is  done  by  the  flywheel  until  the  weight 
has  been  lifted  to  the  original  position,  the  same  cycle  is  then 
repeated.  During  one  half  cycle  the  power  is  positive  or  energy 
is  put  into  the  flywheel  while  during  the  next  half  cycle  the  power 
is  negative  or  energy  is  being  taken  from  the  flywheel,  so  that  the 
average  power  is  zero. 

243.  Examples  of  Inductive  and  Non-inductive  Circuits.— 

The  coefficient  of  self  induction  L  =  —j-  10  ~8  so  that,  to  have  a 

14. 


210     PRINCIPLES  OF  ELECTRICAL  ENGINEERING    [CHAP, 


large  inductance,  a  circuit  must  be  linked  by  a  large  flux  0  for 
a  given  current  7.  The  inductance  of  coil  B  Fig.  233  is  much 
greater  than  that  of  the  duplicate  coil  A  because  the  flux  <j> 
has  been  greatly  increased  by  the  addition  of  the  iron  core  C. 

One  form  of  adjustable  inductance  is  shown  in  Fig.   234. 
If  the  iron  cross  piece  mn  is  brought  nearer  to  the  poles  pq,  the 


Wood 


FIG.  233. — Inductive  circuits. 


reluctance  of  the  magnetic  circuit  is  decreased,  so  that  a  larger 
flux  0  is  produced  by  a  given  current  /  and  the  inductance  is 
thereby  increased. 

An  incandescent  lamp  filament  has  an  inductance  which  is 
negligible  compared  with  its  resistance.     The  number  of  turns 


FIG.  234. — Adjustable  inductance. 

linked  is  small,  while  the  flux  <f>  has  to  pass  through  a  path  con- 
taining no  iron  and  moreover  is  produced  by  an  exciting  coil 
having  only  two  or  three  turns,  so  that  0  is  small  and  L  = 
(/10/7)  10~8  is  negligible.     The  resistance  on  the  other  hand  is 
high,  that  of  a  16  candle-power  carbon  lamp  being  about  200  ohms. 


ART.  244] 


INDUCTIVE  CIRCUITS 


211 


_  While  a  transmission  line  has  only  one  turn,  its  inductance 
is  not  negligible  because  that  one  turn  is  very  long  and  is  linked 
by  a  large  flux  <f>  particularly  if  the  wires  are  spaced  far  apart 
because  then,  as  shown  in  Fig.  235,  there  is  room  for  a  large 
flux  to  pass  between  the  wires. 


Diagram  C 


Diagram   B 

Wires  far  apart.  Wires  close  together. 

FIG.  235. — Flux  linking  a  transmission  line. 

A  simple  long  loop  such  as  that  in  Fig.  236  has  a  negligible 
inductance  because  there  is  no  room  for  flux  to  pass  between  the 
wires.  Non-inductive  resistances  are  made  in  the  form  of  a 
long  narrow  loop  and  are  then  coiled  up  for  convenience  as  shown 


A  B 

FIG.  236. — Non-inductive  resistance. 

in  diagram  B.     The  resistance  between  a  and  b  may  be  large, 
but  the  inductance  is  negligible. 

244.  Voltage,  Current  and  Power  in  Resistance  Circuits. — If 
an  alternating  voltage  is  applied  to  a  non-inductive  circuit  of 
resistance  R  then,  since  there  is  no  e.m.f  of  self  induction  oppos- 
ing the  change  of  current,  the  current  i  at  any  instant  =  e/R 


212      PRINCIPLES  OF  ELECTRICAL  ENGINEERING    [CHAP,  xxix 


and  increases  and  decreases  with  the  voltage,  or  is  in  phase  with 
the  voltage,  as  shown  in  Fig.  237. 

The  power  in  such  a  circuit  is  zero  at  the  instants  a,  b,  c  and  d 
when  both  voltage  and  current  are  zero  but  is  positive  at  all  other 
instants,  that  is  energy  is  put  into  the  circuit  but  none  is  taken  out 
again.  The  average  power 

=  the  average  value  of  ei 

=  the  average  value  of  Em  sin  6  X  Im  sin  6 

=  Emlm  (average  value  of  sin2  0) 


~     , 
Em 
V2X 


see 
Im 

V2 


198 


=  El  and  is  also    =  PR  since  E  =  IR 


FIG.  237. — Voltage  e,  current  i  and  power  e  X  i  in  a  resistance  circuit. 

where  E  and  /  are  effective  values,  see  page  198.  Thus,  in  a  non- 
inductive  circuit,  the  power  is  the  product  of  the  effective  voltage 
and  the  effective  current. 

245.  Resistance  and  Inductance  in  Series. — If  an  alternating 
current  /  is  flowing  in  a  circuit  with  a  resistance  R  and  a  reactance 
X  in  series,  as  shown  in  Fig.  238,  then  (alternating  voltages  Er  = 
IR  and  Ex  =  IX  will  be  found  across  the  two  parts  of  the  circuit. 
The  applied  voltage  E  is  the  vector  sum  of  the  two  components 
Er  and  Ex  and  may  be  determined  as  follows: 

A  vector  /  is  drawn  in  any  direction. 

A  vector  Er  =  IR  is  drawn  to  scale  and  in  phase  with  /,  see 
above. 

A  vector  Ex  =  IX  is  drawn  to  scale  in  such  a  direction  that  7 
lags  Ex  by  90  degrees,  see  page  207. 


ART.  246]  INDUCTIVE  CIRCUITS 

Then  E  =  the  vector  sum  of  Er  and  Ex 


213 


The  current  now  lags  the  applied  voltage  by  an  angle  a,  as 
shown  by  the  vectors  and  by  the  curves  in  Fig.  238.  The  power 
curve  e  X  i  is  also  shown  from  which  it  may  be  seen  that,  although 
the  power  is  negative  during  a  portion  of  the  cycle  yet  the  average 
power  is  positive. 


FIG.  238. — Voltage,  current  and  power  in  a  circuit  which  has  resistance  and 
inductance  in  series. 

The  voltage  E  is  the  resultant  of  two  components  one  of  which 
Er  =  E  cos  a  is  in  phase  with  7  while  the  other  component  Ex  = 
E  sin  a  leads  7  by  90  degrees.  The  average  power  due  to  the  in 
phase  component  =  (E  cos  a)  X  7,  see  page  212;  that  due  to  the 
other  component  is  zero,  see  page  209,  so  that  the  total  average 
power  =  El  cos  a. 

246.  The   power  factor   in  an  alternating-current    circuit   is 

actual  power 
denned  as  the  ratio 

apparent  power 

In  any  circuit  in  which  the  phase  angle  between  the  voltage  E 
and  the  current  7  is  a  degrees  then 


214     PRINCIPLES  OF  ELECTRICAL  ENGINEERING    [CHAP,  xxix 

the  apparent  power  =  El  watts 

but  the  actual  power  =  El  cos  a  watts 

,  El  COS  a 

therefore  the  power  factor  FT 

=  cos  a 
and  can  never  be  greater  than  unity. 

If  a  resistance  of  25  ohms  and  an  inductive  reactance  of  50  ohms  at  60 
cycles  are  put  in  series  across  110  volts;  find  the  current,  the  voltages  across 
the  two  parts  of  the  circuit  and  the  power  in  the  circuit  at  30,  60  and  120 
cycles. 

The  work  is  carried  out  in  tabular  form  as  follows : 


Frequency 

R  ohms 

X  =  27T/L 

VR* 

+  X2 

/ 

Er 

Ex       cos  a 

Watts 

30 

25 

25 

35 

.4 

!3.10 

78 

78      0 

71 

240 

60 

25 

50 

56 

0 

1.96 

49 

!  98 

0 

44 

96 

120 

25 

100 

103 

.0 

ra 

27 

107      0 

24  ! 

29 

247.  The  Wattmeter. — Power  in  alternating-current   circuits 
may  be  measured  by  means  of  an  electrodynamometer  type  of 


Wattmeter 
I 


FIG.  239. — Wattmeter  connections. 

instrument  called  a  wattmeter,  constructed  as  described  on  page 
199,  and  connected  as  shown  in  Fig.  239.  The  line  current  /  is 
passed  through  the  stationary  coils  A,  while  the  current  which 
passes  through  the  moving  coil  C  is  proportional  to  the  voltage  E 
and  is  in  phase  with  it  since  the  inductance  of  the  coil  C  is  negligi- 
ble compared  with  the  additional  resistance  r. 

Since  the  moving  coil  C  is  carrying  current  and  is  in  a  magnetic 
field  it  is  acted  on  by  a  force  tending  to  turn  it  about  a  vertical 
axis  and  this  force  is  proportional  to  the  current  in  the  coil  and  to 
the  strength  of  the  magnetic  field.  When  the  instrument  is 
connected  as  shown  in  Fig.  239,  the  magnetic  field  is  proportional 
to  the  current  /  while  the  current  in  the  moving  coil  is  propor- 
tional to  the  voltage  E,  and  the  average  turning  force  is  proper- 


AUT.  248]  INDUCTIVE  CIRCUITS  215 

tional  to  the  average  value  of  e  X  i  or  to  El  cos  a,  the  average 
power  in  the  circuit. 

If  in  the  circuit  shown  in  Fig.  239 
E  =  100  volts 
7  =  50  amp. 
W  =  4000  watts,  measured  by  a  wattmeter. 

actual  power 
then  the  power  factor  of  the  circuit  =  - 

apparent  power 

4000 

~  100  X  50 
.=  0.8 

and  the  phase  angle  between  current  and  voltage  is  the  angle  whose  cosine  is 
0.8  or  is  37  degrees. 

248.  Transmission  Line  Regulation  and  Losses. — A  transmis- 
sion line  has  resistance  and  inductance  and  may  therefore  be 


/     b 
A  B 

FIG.  240. — Vector  diagram  for  a  transmission  line. 

represented  as  in  Fig.  240.  Eg,  the  voltage  at  the  generating 
station,  is  the  vector  sum  of  the  terminal  voltage  Ety  the  resistance 
drop  IR  and  the  reactance  drop  IX;  the  phase  relation  between 
these  voltages  is  shown  in  diagram  B. 

A  vector  /  is  drawn  in  any  direction. 

A  vector  Et  is  drawn  to  scale  equal  to  the  receiver  voltage,  the 
angle  a  depending  on  the  resistance  and  inductance  of  the  load 
connected  to  the  line; 

A  vector  Er  =  IR  is  drawn  to  scale  in  phase  with  /. 

A  vector  Ex  =  IX  is  drawn  to  scale  in  such  a  direction  that  / 
lags  Ex  by  90  degrees. 

The  vector  Eg  is  the  vector  sum  of  Et,  Er  and  Ex  and  may  be 
scaled  off  or  determined  by  calculation. 

Since  there  is  no  power  loss  in  the  inductance  of  the  line,  the 
total  loss  is  in  the  resistance  and  is  equal  to  PR  watts. 

Values  of  line  resistance  and  line  reactance  are  generally  given 
in  ohms  per  mile  as  in  the  following  table: 


216     PRINCIPLES  OF  ELECTRICAL  ENGINEERING    [CHAP,  xxix 


Size  of  wire 

Resistance 

Inductive 
reactance  per  mile  of  wire 

at  GO  cycles 

B.  and  S. 
gauge 

Cir.  mils 

Ohms  per 
,   mile  of   wire 

24" 

48" 

72" 

96"  spacing  in 
inches 

0000 

211600 

0.258 

0.594 

0.678 

0.728 

0.763 

000 

167800 

0.326 

0.608 

0.692 

0.742 

0.776 

00 

133100 

0.411 

0.622 

0.706 

0.756 

0.790 

0 

105500 

0.518 

0.637 

0.721 

0.770 

0.804 

1 

83690 

0.653 

0.650 

0.735 

0.784 

0.819 

2 

66370 

0.824 

0.665 

0.749 

0.797 

0.833 

4 

41740 

1.309 

0.693 

0.777 

0.826 

0.860 

6 

26250 

2.082 

0.721 

0.805 

0  .  854 

0.889 

75  kw.  at  2200  volts  and  60  cycles  has  to  be  delivered  at  the  end  of  a  5- 
mile  line,  the  size  of  wire  being  No.  0  B.  and  S.  gauge  and  the  spacing  48 
in.  Find  the  voltage  in  the  generating  station  and  the  power  loss  in  the 
line  if  the  load  current  is  lagging  and  the  power  factor  is  0.8. 

watts  =  Et  X  I  X  cos  a,  see  page  214, 
or     75  X  1000  =  2200  X  /  X  0.8 
and  the  current  =  42.5  amp. 
the    resistance  =  0.518  ohms  per  mile  of  wire 

=  5.18  ohms  for  a  5-mile  line 
the    reactance  =  0.721  ohms  per  mile  of  wire  at  60  cycles 

=  7.21  ohms  for  a  5-mile  line 
IR  =  42.5  X  5.18  =  220  volts 
IX  =  42.5  X  7.21  =  307  volts 
ab,      Fig.    240  =  (2200  X  0.8)  +  220  =  1980 
be,      Fig.    240  =  (2200  X  0.6)  +  307  =  1627 

E0  =  V(a&)2+  (be)2  =  \/19802~+l6272 
=  2560  volts 

the  power  factor  at  the  generating  station  =  —  =  0550  =  ®-^5 

the  power  put  into  the  line  =  2560  X  42.5  X  0.775  =  84.4  kw. 

the  power  delivered  =  75  kw. 

the  loss  in  the  line  =  84.4  —  75  =  9.4  kw. 

and  this  is  equal  to  PR  =  (42.5)2  X  5.18  =  9.4  kw. 

249.  Resistance  and  Inductance  in  Parallel. — If  an  alternating 
voltage  E  is  applied  to  a  circuit  which  has  a  resistance  R  and  a 
reactance  X  in  parallel,  as  shown  in  Fig.  241,  then  a  current 
Ir  =  E/R  will  flow  through  the  resistance  and  will  be  in  phase 
with  E  while  a  current  Ix  =  E/X  will  flow  through  the  reactance 
and  will  lag  E  by  90  degrees. 

These  two  currents  are  drawn  to  scale  in  diagram  B  and  the 
total  current  /  is  the  vector  sum  of  Ir  and  Ix  and 


ART.  249] 


INDUCTIVE  CIRCUITS 


217 


If  a  resistance  of  25  ohms  and  an  inductive  reactance  of  50  ohms  at  60 
cycles  are  put  in  parallel  across  110  volts,  find  the  current  in  each  part  of  the 
circuit  and  also  the  total  current  at  30,  60  and  120  cycles. 


A  B 

FIG.  241. — Vector  diagram  for  a  parallel  circuit. 

The  work  is  carried  out  in  tabular  form  as  follows : 


Frequency 

R  ohms            X  =2*/L           Ir          Ix    \     I     \     cos  a        Watts 

30 

60 
120 

25                  25            4.4 
25                  50            4.4 
25                100            4.4 

4.4 
2.2 
1.1 

6.200.710 
4.930.890 
4.560.965 

485 
485 
485 

CHAPTER  XXX 
CAPACITY  CIRCUITS 

250.  Condensers. — Two  conducting  bodies  separated  by  insu- 
lating material  form  what  is  known  as  an  electrostatic  condenser. 
In  diagram  A,  Fig.  242,  a  and  b,  two  plates  of  a  condenser,  are 
at  the  same  potential.  When  the  switch  k  is  closed  a  momentary 
current  i  passes  in  the  direction  of  the  arrows  in  diagram  B  and 
the  condenser  is  said  to  be  charged;  the  potential  of  plate  a  is 
raised  to  that  of  the  positive  line  terminal,  the  potential  of  plate 
b  is  lowered  to  that  of  the  negative  line  terminal  and  the  differ- 
ence of  potential  between  the  plates  becomes  equal  to  the  line 
voltage. 


A  -  Condeiiser  B-  Condenser  Charging        C- Condenser  Discharging 

without  Charge  and  Storing  Electricity  and  giving  up  Electricity 

FIG.  242. — Charge  and  discharge  of  a  condenser. 

If  the  switch  k  is  now  opened,  the  voltage  between  the  plates 
remains  unchanged  and  a  quantity,  or  charge,  of  electricity  re- 
mains stored  in  the  condenser. 

To  make  the  condenser  give  up  its  charge,  the  insulated  plates 
must  be  connected  by  a  conducting  material,  such  as  the  wire  d 
in  diagram  C,  so  as  to  bring  them  to  the  same  potential.  When 
this  is  done,  a  momentary  current  passes  in  the  direction  shown, 
from  the  positive  to  the  negative  plate. 

The  quantity  of  electricity  stored  in  a  condenser,  called  the 
charge,  is  equal  to  the  average  current  flowing  into  the  condenser 

multiplied  by  the  time  during  which  it  flows  or  is  equal  to  I  idt 

where  i  is  the  charging  current  at  any  instant.  In  any  condenser 
this  charge  is  found  to  be  directly  proportional  to  the  applied 
voltage  or 

q  =  Ce 
218 


ART.  251]  CAPACITY  CIRCUITS  219 

where  q  is  the  charge  in  coulombs  (amperes  X  seconds) 
e    is  the  applied  voltage 

C  is   a   constant   called   the   capacity   of  the   condenser 
and  is  expressed  in  farads. 

A  condenser  of  1  farad  capacity  will  hold  a  charge  of  1  coulomb 
if  a  difference  of  potential  of  1  volt  is  applied  between  the  plates. 

The  capacity  of  a  condenser  of  given  dimensions  is  found  to 
depend  on  the  insulating  material,  or  dielectric,  between  the 
plates.  If  a  condenser  with  air  as  dielectric  has  a  capacity  of 
F  farads  then  the  capacity  becomes  equal  to  kF  farads  when 
another  dielectric  is  used.  The  constant  k  is  called  the  specific 
inductive  capacity  of  the  material.  Average  values  of  k  are 
given  in  the  following  table  for  the  materials  generally  used 
in  commercial  condensers. 

Material  Specific  inductive  capacity 

Air  1 

Glass  4      (varies  considerably  with  the 

quality  of  the  glass) 
\       Mica  6 

Paraffined  paper  2 

251.  Capacity  Circuits  with  Direct  and  with  Alternating 
Currents. — A  capacity  circuit  is  one  which  contains  a  condenser. 


y  y 

A  B 

FIG.  243. — Flow  of  current  in  a  capacity  circuit. 

If  a  constant  e.m.f.  is  applied  across  the  terminals  of  the  circuit 
shown  in  Fig.  243,  then  a  momentary  current  will  flow  in  the  direc- 
tion shown  in  diagram  A  to  charge  the  condenser  but  current 
will  not  flow  continuously  since  the  circuit  is  broken  by  the  insu- 
lating material  between  the  plates. 

If  the  applied  voltage  is  now  reversed,  current  will  flow  in  the 
direction  shown  in  diagram  B  until  the  condenser  has  given  up 
its  charge,  and  will  continue  to  flow  in  this  direction  until  the 
condenser  is  re-charged  in  the  opposite  direction.  If  then  the 
applied  voltage  is  alternating,  a  charging  current  will  flow  in  and 


220      PRINCIPLES  OF  ELECTRICAL  ENGINEERING     [CHAP,  xxx 

out  of  the  wires  x  and  y  with  a  frequency  which  is  the  same  as 
that  of  the  applied  voltage,  and  the  lamps  L  will  light  up  if 
sufficient  current  flows  to  heat  the  filaments. 

The  greater  the  capacity  of  a  condenser,  the  more  electricity 
it  can  hold,  and  the  larger  the  charging  current  that  passes 
through  the  connecting  wires.  Furthermore,  the  greater  the 
frequency  of  the  applied  voltage,  the  shorter  the  time  available 
in  which  to  charge  the  condenser,  and  therefore  the  larger  the 
current  that  must  flow.  With  a  given  applied  alternating  vol- 
tage, and  therefore  with  a  definite  alternating  charge  (since  q  = 
Ce),  the  alternating  current  i  is  proportional  to  the  capacity 

and  to  the  frequency  or 

\ 

i  =  a  const.  X  C  X  / 

the  constant  will  be  determined  later. 

252.  Phase  Relation  between  Voltage  and  Current  in  Capacity 
Circuits. — If  the  voltage  applied  to  the  condenser  in  Fig.  244  be 
represented  by  curve  E  then  the  charge,  which  is  proportional  to 
the  voltage,  is  represented  by  the  curve  Q;  the  condenser  is 


FIG.  244. — Phase  relation  between  the  voltage  and  current  in  a  capacity 

circuit. 

charged  alternately  in  opposite  directions,  thus  between  the 
instants  m  and  n  the  plate  a  is  positive  while  between  n  and  p 
the  plate  a  is  negative. 

At  the  instants  q  and  r  the.  charge  in  the  condenser  is  not  chang- 
ing, the  currents  in  the  leads  x  and  y  must  therefore  be  zero. 

Between  m  and  q  the  voltage  and  the  charge  are  increasing  and 
current  flows  in  the  positive  direction,  from  the  positive  to  the 
negative  terminal  as  shown  in  diagram  A,  until,  at  the  instant 
q,  the  charge  is  complete  and  the  current  has  become  zero;  this 
gives  the  part  fq  of  the  current  curve,  see  diagram  C. 

Between  q  and  n  the  voltage  and  the  charge  are  decreasing  so 
that  current  must  now  be  flowing  out  of  the  condenser  or  in  the 


ART.  253]  CAPACITY  CIRCUITS.  221 

negative  direction  as  shown  in  diagram  B;  this  gives  the  part 
qg  of  the  current  curve. 

During  the  next  half  cycle  between  n  and  p  the  condenser 
charges  and  discharges  in  the  opposite  direction,  so  that  the  cur- 
rent curve  gh  is  the  same  as  the  curve  fg  except  that  the  sign  is 
reversed. 

From  these  curves  it  may  be  seen  that  the  current  leads  the  vol- 
tage by  90  degrees. 

In  the  above  discussion  of  phase  relation  between  current  and 
voltage  in  capacity  circuits  it  is  assumed  that  the  current  has  been 
flowing  for  a  few  seconds.  It  is  obvious  that,  at  the  instant  the 
switch  in  a  circuit  is  closed,  the  current  in  that  circuit  must  be 
zero  no  matter  what  value  the  e.m.f.  may  have,  so  that  the  cur- 
rent waves  are  generally  abnormal  for  a  few  cycles  after  the  clos- 
ing of  the  switch,  but  they  gradually  change  and  become  regular 
waves  leading  the  e.m.f.  by  90  degrees. 

253.  Voltage  and  Current  Relations  in  Capacity  Circuits.  —  The 
charge  in  the  condenser  shown  in  Fig.  244  changes  from  zero  to 
Qm  =  CEm  coulombs  in  the  time  of  one-quarter  of  a  cycle,  or 
in  1/4/  seconds,  so  that,  since  charge  =  average  current  X  time, 

therefore  Qm  =  CEm  =  Iav  X  j-> 

and  Iav,  the  average  charging  current  =  4fCEm  amp. 

7T 

Now  the  maximum  charging  current  Im  =  Iav  X  <->>  see  page  197. 

=  *  X  ±fCEm 


therefore  /,  the  effective  current  in  a  capacity  circuit 

=  2-rrfCE  where  E  is  the  effective  voltage. 

The  physical  meaning  of  this  equation  was  explained  on 
page  220. 

In  direct-current  circuits  E  =  IR]  in  capacity  circuits  E  =  IX 
where  X,  called  the  capacity  reactance,  is  expressed  in  ohms  and 

is  numerically  equal  to 


An  alternating  e.m.f.  of  110  volts  sends  2.2  amp.  through  a  capacity 
circuit  at  60  cycles.  Find  the  reactance  at  60  cycles  and  find  also  the  capac- 
ity of  the  condenser. 


222      PRINCIPLES  OF  ELECTRICAL  ENGINEERING    [CHAP,  xxx 

X,  the  reactance  =  E/I 

=  110/2.2  =  50  ohms. 

C,  the  capacity  =  ^.^ 

=  2^60  50  =  ^  ^  ^~5  ^ara(^s  =  0-53  microfarads. 

If  the  voltage  applied  to  the  above  circuit  is  kept  constant  at  110,  find  the 
current  that  will  flow  through  the  capacity  at  30,  60,  90  and  120  cycles. 

Xy  the  reactance  =  O^TTY  =  50  ohms  at  60  cycles,  from  last  problem 

=  100  ohms  at  30  cycles 
=  33.3  ohms  at  90  cycles 
=  25  ohms  at  120  cycles 

7,  the  current  =  E/X=  110/100  =  1.1  amp.  at  30  cycles 
=  110/50  =  2.2  amp.  at  60  cycles 
=  110/33.3  =  3.3  amp.  at  90  cycles 
=  110/25  =  4,4  amp.  at  120  cycles 

254.  Parallel  Plate  Condenser. — As  shown  in  the  last  problem, 
a  condenser  with  a  capacity  of  0.53  microfarads  will  take  a  cur- 


FIG.  245. — One  method  of  constructing  condensers. 


rent  of  2.2  amp.  at  110  volts  and  60  cycles.     It  is  desirable  to 
know  the  approximate  dimensions  of  such  a  condenser. 

The  capacity  of  a  parallel  plate  condenser  is  given  by  the 
formula 

C  in  farads  =       x        ~        X  T 


where  A  is  the  area  of  the  active  surface  of  one  plate  in  sq.  cm. 
t  is  the  distance  between  plates  in  cm. 
k  is  the  specific  inductive  capacity,  see  page  219. 

A  condenser  constructed  as  in  Fig.  245  has  plates  of  tin  foil  which  are  40 
ft.  long  and  3  in.  wide  and  are  separated  by  paraffined  paper  0.0025  in.  thick. 
Since  both  sides  of  each  plate  are  active, 

A=2X40X12X3=    2,880  sq.  in. 
=  18,600  sq.  cm. 


ART.  256]  CAPACITY  CIRCUITS  223 

t  =  0.0025  in.  =  0.0063  cm. 
k  =  2.0 

1  1  18600 

g  X1QU  X  X  2 


=  0.53  X  10~6  farads 
=  0.53  microfarads 

Such  a  condenser  will  go  into  a  tin  case  1.75  in.  square  by  4  in.  deep. 

255.  Power  in  Capacity  Circuits.  —  The  power  in  a  circuit  at 
any  instant  is  the  product  of  e  and  i  the  voltage  and  the  current  at 
that  instant.  In  a  capacity  circuit  the  current  leads  the  applied 
voltage  by  90  degrees  and  the  curves  representing  e,  i  and  ei  are 
shown  in  Fig.  246.  This  latter  curve  is  obtained  by  multiplying 
together  corresponding  values  of  e  and  i  at  different  instants; 


FIG.  246. — Voltage  e,  current  i  and  power  e  X  i  in  a  capacity  circuit. 

at  m  and  n  the  voltage  and  therefore  the  power  are  zero;  the 
power  is  also  zero  at  instants  q  and  r  when  the  current  is  zero. 
Between  m  and  q  energy  is  stored  in  the  condenser  while  between 
q  and  n  the  same  energy  is  given  up  by  the  condenser,  so  that  the 
average  value  of  the  energy  used  is  zero  and  so  also  is  the  average 
power  in  the  circuit. 

256.  The  Formulae  used  in  Circuit  Problems  are : 
Resistance  Circuit: 
E  =  IR 

current  is  in  phase  with  voltage,  see  page  211 
power  =  EI  watts. 
Circuit  with  Inductive  Reactance  : 
E  =  IX  where  X  =  2irfL,  see  page  208 
current  lags  voltage  by  90  degrees,  see  page  207 
power  is  zero. 


224      PRINCIPLES  OF  ELECTRICAL  ENGINEERING     [CHAP,  xxx 


Circuit  with  Capacity  Reactance  : 

E  =  IX  where  X  =      -n'  see  PaSe  221 


current  leads  voltage  by  90  degrees,  see  page  220 
power  is  zero. 


i 

—  r—  ITOW^, 

TTt 


Besistauce 
Circuit 


Inductive 
Circuit 

FIG.  247. 


Capacity 
Circuit 


257.  Resistance,  Inductance  and  Capacity  in  Series.  —  In  the 
solution  of  such  a  circuit  as  that  shown  in  Fig.  248,  the  current 
vector  has  to  be  taken  as  a  basis  for  phase  relation  since  it  is  the 
same  in  all  three  parts  of  the  circuit.  The  voltage  E  is  the  vec- 
tor sum  of  Er,  EI  and  Ec  and  is  determined  as  follows  : 

A  vector  /  is  drawn  in  any  direction. 

A  vector  Er  =  IR  is  drawn  to  scale  in  phase  with  7. 

A  vector  Et  =  IX  \  is  drawn  to  scale  in  such  a  direction  that  7 
lags  EI  by  90  degrees. 

A  vector  Ec  =  IX  c  is  drawn  to  scale  in  such  a  direction  that  7 
leads  Ec  by  90  degrees. 

Then  E  =  the  vector  sum  of  Er,  EI  and  Ec 


=  7  ^+(XX 

and  the  current  will  lead  or  lag  the  applied  voltage  according  as 
Xc  is  greater  or  less  than  Xt. 

When  Xc  =  Xt  the  capacity  and  the  inductive  reactances 
exactly  neutralize  one  another  and  the  current  has  its  maximum 
value  and  is  equal  to  E/R.  The  circuit  is  then  said  to  be  in 
resonance. 


ART.  257] 


CAPACITY  CIRCUITS 


225 


The  inductive  reactance  Xt  is  directly  proportional  to  the  fre- 
quency and  is  equal  to  2irfL  whereas  the  capacity  reactance  Xc  is 
inversely  proportional  to  the  frequency  and  is  equal  to  l/2wfC. 
If  then  in  Fig.  248,  the  voltage  E  across  the  terminals  is  kept  con- 
stant and  the  frequency  is  increased,  Xi  will  increase  and  Xc  will 
decrease  until  when 

Xl  =XC 


or 


and 


27T/L    = 


1 

27T/C 
1 

7r  VLCy 


Er 


Applied 
Voltage 


Frequency 


FIG.  248. — Voltage  and  current  in  a  circuit  with  resistance  R,  inductive 
reactance  Xe  and  capacity  reactance  Xc  in  series. 

the  circuit  is  sa*d  to  be   in  resonance  and  the  current  has  its 
maximum  value. 

The  same  problem  is  found  in  mechanics.  An  alternating 
force  applied  to  a  spring  will  cause  the  spring  to  oscillate.  As 
the  frequency  of  the  applied  force  increases,  the  amplitude  in- 
creases and  reaches  its  maximum  value  when  the  applied  fre- 
quency is  the  same  as  the  natural  frequency  of  vibration  of  the 
spring;  with  further  increase  of  the  frequency,  the  amplitude 
will  decrease.  This  principle  is  made  use  of  in  the  instrument 
shown  in  Fig.  215. 

15 


226      PRINCIPLES  OF  ELECTRICAL  ENGINEERING    [CHAP,  xxx 

The  frequency  /  = j=  is  called  the  frequency  of  resonance 

2?r  A/jLC 

and  is  also  called  the  natural  frequency  of  the  circuit. 

If  a  resistance  of  2  ohms,  an  inductive  reactance  of  12  ohms  at  60  cycles 
and  a  capacity  reactance  of  20  ohms  at  60  cycles  are  put  in  series  across  110 
volts  plot  the  current  7  and  also  the  voltages  Er,  Ei  and  Ec  against  fre- 
quency. 

Several  points  for  these  curves  are  determined  as  follows : 


o 

5         ? 

" 

o 

^ 

3         L     -JS        S 

l"s 

£ 
*** 

§ 

(^ 

e§ 

CM 

|                                  $**                                  J_                                .. 

" 

o4 

|| 

|| 

"*»                           ^^                             e» 

ii 

t-t 

£? 

0 

^*                "4*                 M                *~ 

£ 

£ 

r^ 

^N 

Sj        v 

>       ^ 

' 

20.0 

2 

4.0 

60.0 

-56 

56  + 

1.97 

3.94 

7.9 

118 

40.0 

2 

8.0 

30.0 

-22 

22.1 

4.98 

9.96 

40.0 

150 

60.0 

2 

12.0 

20.0 

-8 

8.24 

13.4 

26.8 

161.0 

268 

80.0 

2 

16.0 

15.0 

1 

2.24 

.49.2 

98.4 

788.0 

738 

100.0 

2 

20.0 

12.0 

8 

8.24 

13.4 

26.8 

268.0 

161 

77.5 

2 

15.5 

15.5  !         0 

2.0 

55.0      110.0 

852.0 

852 

the  frequency  of  resonance  can  be  determined  readily  by  trial  and  is  77.5 
cycles,  because  then  Xi  =  Xc  =  15.5  ohms. 

If  the  circuit  is  in  resonance  and  the  resistance  is  low  then  a 
large  current  will  flow,  and  if  in  addition  the  reactances  are  large 
then  the  voltage  drops  across  these  reactances  are  large  and  may 
have  several  times  the  value  of  the  applied  voltage,  this  result  is 
shown  in  the  above  problem;  the  inductance  coil  and  the  con- 
denser must  be  insulated  to  withstand  852  volts  and  not  merely 
the  applied  110  volts. 

In  a  circuit  which  contains  only  resistance,  E  =  IR 
in  a  circuit  which  contains  only  inductance,  fc  =  IXi 
in    a    circuit  which   contains    only   capacity,  E  =  IX c 
in  a  circuit  which  contains  all  three  in  series,  E  =  IZ 
where  Z,  called  the impedence  of  the  circuit  =  V#2  +  (Xi  —  Xc)2 

258.  Resistance,  Inductance  and  Capacity  in  Parallel. — In 
the  solution  of  such  a  circuit  as  that  shown  in  Fig.  249,  the  voltage 
vector  has  to  be  taken  as  a  basis  for  phase  relation  since  it  is  the 
same  for  all  three  parts  of  the  circuit.  The  current  /  is  the  vector 
sum  of  Ir,  1 1  and  Ic  and  is  determined  as  follows: 

A  vector  E  is  drawn  in  any  direction. 


ART.  258] 


CAPACITY  CIRCUITS 


227 


A  vector  Ir  =  E/R  is  drawn  to  scale  in  phase  with  E. 
A  vector  It  =  E/Xt  is  drawn  to  scale  and  lagging  E  by  90 
degrees. 

A  vector  Ic  =  E/XC  is  drawn  to  scale  and  leading  E  by  90 
degrees. 
Then    /  =  the  vector  sum  of  Ir,  It  and  Ic 


*+(E/Xt-E/Xc)* 


frequency 

FIG.  249. — Circuit  with  resistance,  inductance  and  capacity  in  parallel, 


and  the  current  will  lead  or  lag  the  applied  voltage  according  as 
Ie  is  greater  or  smaller  than  7Z. 

When  the  circuit  is  in  resonance,  Xc=Xl  and  the  current  in 
the  line  has  its  minimum  value  and  is  equal  to  E/R.  If  the  re- 
actances are  then  low  compared  with  the  resistance  R,  the  cur- 
rents Ii  and  Ic  may  be  much  larger  than  the  line  current  I  as 
shown  in  the  following  example. 


228      PRINCIPLES  OF  ELECTRICAL  ENGINEERING     [CHAP,  xxx 


If  a  resistance  of  20  ohms,  ;m  inductive  reactance  of  2.4  ohms  at  60  cycles 
and  a  capacity  reactance  of  4  ohms  at  60  cycles  are  put  in  parallel  across  110 
volts,  plot  the  currents  in  the  different  parts  of  the  circuit  against  frequency. 

Several  points  for  those  curves  are  determined  as  follows: 


Frequency 

R 

Xi  -  27T/L 

Y              l 

ZwfC 

IT              It 

Ic 

//  -  /„            / 

20.0 

20 

0.8 

12.0 

137 

9 

12S           128  + 

40.0 

20 

1.6 

6.0 

5.5 

69 

18 

51             51.2 

60.0 

20 

2.4 

4.0 

5.5 

46 

28 

18             18.8 

77.5 

20 

3.1 

3.1 

5.5 

35 

35 

0              5.5 

80.0 

20 

3.2 

3.0 

5.5 

34 

37 

-  3              6.2 

100.0 

20 

4.0 

2.4 

5.5 

28 

46 

-18             18.8 

CHAPTER  XXXI 
ALTERNATORS 

259.  Alternator  Construction. — The  essential  parts  of  a  re- 
volving field  type  of  alternator  are  shown  in  Fig.  250.     The 


E.  m.  f.  wave 

FIG.  250.— Revolving-field  type  of  alternator. 


Si 


FIG.  251. — Winding  diagram. 


FIG.  252.— Alternator  coil. 

stationary  part  which  carries  the  conductors  that  are  cut  by  the 
revolving  field  is  called  the  stator;  the  revolving  field  system  is 
culled  the  rotor. 

229 


230     PRINCIPLES  OF  ELECTRICAL  ENGINEERING    [CHAP,  xxxi 

The  stator  core  B  is  built  up  of  soft  steel  laminations  and  has 
slots  on  the  inner  periphery  in  which  the  stator  coils  are  placed. 
One  type  of  coil  is  shown  in  Fig.  252  and  consists  of  several  turns 
of  copper  wire  which  are  insulated  from  one  another  and  are  then 
taped  up  with  cotton  and  other  such  insulating  material.  The 
machine  shown  in  Fig.  250  has  four  of  these  coils  which  are  con- 
nected in  series  so  that  their  voltages  add  up. 

Since  a  connection  diagram  such  as  Fig.  250  shows  only  one 
end  of  the  machine,  it  is  found  desirable  in  practice  to  show  the 
coils  and  connections  by  means  of  a  developed  diagram  such  as 
Fig.  251;  this  diagram  shows  what  would  be  obtained  if  the 
winding  in  Fig.  250  were  split  at  xy  and  then  flattened  out  on  a 
plane;  the  two  diagrams  are  lettered  similarly. 

The  voltage  between  the  terminals  Si  and  F\  varies  as  shown 
in  Fig.  250  and  goes  through  four  cycles  per  revolution. 

260.  Two-phase  Alternator. — In  order  to  utilize  more  of  the 
stator  surface,  a  duplicate  winding  B  is  placed  on  the  stator  as 


s« 


FIG.  253. — Two-phase  alternator. 

shown  in  Fig.  253.  This  machine  has  twice  as  many  conductors 
as  that  in  Fig.  250,  but  if  the  coils  A  and  B  are  connected  in 
series,  it  will  be  found  that  the  voltage  of  the  machine  has  not 
been  doubled  but  has  been  increased  only  41  per  cent. 

It  was  pointed  out  on  page  200  that  the  distance  between  two 
adjacent  like  poles  is  360  electrical  degrees,  therefore  the  distance 
between  similar  points  on  windings  A  and  B  is  90  electrical 
degrees.  If  then  the  voltage  generated  in  the  four  coils  A  in 
series  is  represented  by  the  curve  Ea)  Fig.  254,  that  generated 
in  the  four  coils  B  in  series  has  the  same  magnitude  but  lags  Ea  by 
90  degrees  and  is  therefore  represented  by  the  curve  Eb;  when  the 
poles  are  in  the  position  shown,  for  example,  the  voltage  in  the 


ART.  261] 


ALTERNATORS 


231 


winding  A  is  a  maximum  while  that  in  B  is  zero,  these  values  are 
obtained  at  the  instant  c,  Fig.  254. 

The  resultant  voltage  when  the  two  windings  are  connected  in 
series  is  the  vector  sum  of  E\  and  E%  and  is  equal  to  ^J2E  =  1.414J£ 
and  if  I  is  the  maximum  safe  current  the  conductors  of  the 
winding  can  carry  then  the  maximum  output  is  1.414  El  watts. 
It  is  therefore  desirable  to  use  the  two  windings  A  and  B  as  if 
they  belonged  to  separate  alternators  and  then,  by  dividing  up 
the  load  between  them  as  shown  diagrammatically  in  Fig.  256, 
each  winding  can  be  made  to  deliver  El  watts  or  the  whole 
machine  be  made  to  deliver  2EI  watts. 


FIG.  254. — Voltage  curves  of  a 
two-phase  alternator. 


FIG.  255.— 
Voltage  vector 
diagram  for  a 
two-phase  al- 
ternator. 


FIG.  256. — Diagram- 
matic representation  of 
a  two-phase  alternator. 


Since  the  voltage  of  winding  B  is  out  of  phase  with  that  of 
winding  A,  the  machine  operating  as  shown  in  Fig.  253  gives  two 
phases  of  voltage  and  is  called  a  two-phase  machine^  whereas  that 
shown  in  Fig.  250  is  a  single-phase  machine.  The  former  machine 
requires  four  wires  for  the  load  while  the  latter  requires  only  two. 

261.  Three-phase  Alternators. — If  three  similar  and  independ- 
ent single-phase  stators  are  mounted  beside  one  another  as  in 
Fig.  257  in  such  a  way  that  their  conductors  are  cut  by  the  same 
revolving  field,  then  three  separate  single-phase  e.m.fs.  may  be 
obtained,  one  from  each  winding.  If  further  these  stators  are 
mounted  so  that  Si,  Sz  and  S3,  the  starts  of  the  windings  of  the 
three  phases,  are  spaced  120  electrical  degrees  apart,  then  the 
e.m.f.  in  winding  B  will  reach  a  maximum  120  degrees  after  that 
in  winding  A  has  reached  its  maximum  value,  and  the  e.m.f.  in 
winding  C  will  lag  that  in  winding  B  by  120  degrees,  as  shown  in 
Fig.  259. 


232      PRINCIPLES  OF  ELECTRICAL  ENGINEERING    [CHAP,  xxxi 


A  B  C_ 

*\~ 


st 


FIG.  257. — Three  single-phase  stators. 


FIG.  258. — Three-phase  alternator. 


FIG.  259. — Voltage  curves  of  a  three- 
phase  alternator. 


FIG.  260. — Voltage  vector 
diagram  for  a  three-phase  al- 
ternator. 


ART.  262] 


ALTERNATORS 


233 


In  practice  the  three  windings  are  placed  on  the  same  core  as  in 
Fig.  258,  but  it  must  be  noted  that  in  the  resulting  three-phase 
machine  the  windings  are  independent  of  one  another  and  supply 
distinct  and  independent  e.m.fs.  to  three  distinct  and  independent 
circuits  so  that  the  machine  is  exactly  equivalent  to  three  separate 
single-phase  machines  and  is  therefore  called  a  three-phase  ma- 
chine. Part  of  the  winding  for  such  a  machine  is  shown  in  Fig.  261 . 


FIG.  261. — Part  of  the  stator  of  a  large  three-phase  alternator. 

262.  Y-Connection. — A  three-phase  machine  is  conveniently 
represented  by  a  diagram  such  as  that  in  Fig.  262,  the  three  vec- 
tors in  Fig.  260  being  replaced  by  three  separate  and  independent 
windings ;  such  a  machine  has  six  terminals  and  six  leads,  two  for 
each  phase. 

In  order  to  reduce  the  number  of  leads,  the  three  return  wires  a2, 
62  and  c2  may  be  connected  together  to  form  a  single  wire  n.  The 
current  in  this  wire  at  any  instant  is  therefore  the  sum  of  ii,  iz  and 
23,  the  currents  in  the  three  phases.  But  it  may  be  seen  from 
diagram  B  that,  at  any  instant,  the  sum  of  these  three  currents  is 
zero;  at  instant  a  for  example  ii  is  equal  and  opposite  to  i%  +  i$ 
while  at  instant  b,  ii  is  equal  and  opposite  to  i3  and  iz  is  zero,  the 
wire  n  therefore  carries  no  current  and  may  be  dispensed  with. 
The  resultant  connection,  shown  in  Fig.  263,  is  called  the  Y- 
connection  and  requires  only  three  leads  to  supply  the  load,  one 
lead  always  acting  as  the  return  for  the  other  two. 

263.  Delta-connection. — Another  method  of  connecting  the 
three  windings  of  a  three-phase  machine  is  shown  diagrammat- 
ically  in  Fig.  264,  the  windings  being  connected  in  series  in  the 


234     PRINCIPLES  OF  ELECTRICAL  ENGINEERING    [CHAP,  xxxi 


F, 


fe         J2 

?~     t      A. 


Voltage  Curves  of  a  Three  Phase 
5  Machine 


\/ 


JOO 


Current  Curves  of  a  Three  Phase 
Machine 


FIG.  262. — Diagrammatic  representation  of  a  three-phase  machine. 


FIG.  263.— Y-Connection. 


FIG.  264. — Delta-connection. 


ART.  264] 


ALTERNATORS 


235 


following  order  SiF2,S2F3,  S^Fi.  The  wires  making  these  connec- 
tions may  be  shortened  and  the  terminals  connected  directly  to 
one  another  as  shown  in  diagram  B,  the  slope  of  the  vectors  being 
unchanged.  On  account  of  the  appearance  of  this  latter  diagram, 
this  three-phase  connection  is  called  the  delta-connection. 

Although  the  winding  has  been  closed  on  itself,  no  current  flows 
through  this  closed  circuit.  The  resultant  voltage  in  the  closed 
circuit  is  the  sum  of  the  voltages  in  the  three  phases,  but  it  may  be 
seen  from  diagram  A,  Fig.  262,  that,  at  any  instant,  e\  +  e2  +  e3 
is  zero,  the  voltage  in  one  phase  being  always  equal  and  opposite 
to  the  sum  of  the  voltages  in  the  other  two  phases.  If,  however, 
an  external  circuit  is  connected  between  any  two  leads,  then  the 
voltage  across  that  circuit  will  be  E,  the  voltage  of  one  phase, 
and  current  will  flow  through  the  circuit. 

264.  Voltages,  Currents  and  Power  in  a  Y-Connected  Machine. 
—If  two  coils  SiFi  and  $2/^2  are  connected  as  shown  in  Fig.  265 


f\ 


Fz 


FIG.  265. 


FIG.  266.  —  The  vector  difference  between  the  two  voltages 
equal  to  E,  is  Et 


and  E2,  each 


and  the  voltage  E%  lags  EI  by  120  degrees  then  the  resultant 
voltage  Er  is  the  vector  sum  of  EI  and  E2  and  may  be  determined 
as  shown  in  diagram  A. 

If,  however,  the  second  coil  is  connected  backward  as  shown  in 
Fig.  266,  then  the  resultant  voltage  Et  is  no  longer  the  vector 
sum  but  is  the  vector  difference  and  is  obtained  by  reversing  the 


236     PRINCIPLES  OF  ELECTRICAL  ENGINEERING    [CHAP,  xxxi 


vector  to  be  subtracted  and  then  taking  the  sum  as  shown  in 
diagram  B,  where 

Et  =  2E  cos  30 


=  1.73E 

This  latter  connection  is  the  Y  connection,  thus  in  Fig.  263,  FI 
and  F2  are  connected  together  and  the  voltage  between  Si  and  Sz 
=  1.73E.  The  current  It  in  the  line  is  the  same  as  the  current  / 
in  the  winding. 

If  the  current  /  in  each  phase  of  the  machine  lags  the  voltage  E 
of  that  phase  by  an  angle  a  as  shown  in  diagram  B,  Fig.  262,  then 
the  power  in  each  phase  =  El  cos  a  watts 

the  power  in  the  three  phases  =    3EI  cos  a 


3 


cos  a 


cos  a 


since  Et  =   V3#  and  It  =  I. 

265.  Voltages,   Currents   and    Power  in  a   Delta-connected 


Machine.—  If  two  coils 


and  $2^2  are  connected  as  shown  in 


FIG.  267. 


FIG.  268. — The  vector  difference  between  the  two  currents  7:  and  72,  each 
equal  to  /,  is  Ii  =  V3/. 

Fig.  267,  and  the  current  72  lags  7i  by  120  degrees  then  the  result- 
ant current  It  is  the  vector  sum  of  /i  and  72  and  may  be  deter- 
mined as  shown  in  diagram  C. 
•If,  however,  the  second  coil  is  connected  backward  as  shown 


ART.  266] 


ALTERNATORS 


237 


in  Fig.  268,  then  the  resultant  current  It  is  no  longer  the  vector 
sum  but  is  the  vector  difference  as  shown  in  diagram  D  where 

II  =  21  cos  30 
=    V3/ 
=  1.737 

This  latter  connection  is  the  delta- connection,  thus  in  Fig.  264, 
Si  and  Fz  are  connected  together  and  the  current  Ii  in  the  line 
connected  to  that  point  =  1.73/.  The  voltage  Et  is  the  same  as 
the  voltage  E  of  the  winding. 

If  the  current  /  in  each  phase  of  the  machine  lags  the  voltage 
E  of  that  phase  by  an  angle  a  then 
the  power  in  each  phase  =  El  cos  a  watts 

the  power  in  the  three  phases  =  3EI  cos  a 

Ii 

V3, 

=   V3  Etli  cos  a 
since  Et  =  E  and  lt  =   V3i 

With  the  same  current  in  the  line  and  the  same  voltage  between 
lines,  the  power  is  the  same  no  matter  whether  the  machine  is 
connected  Y  or  delta. 

266.  Connection  of  a  Three-phase  Load. — The  load  on  a  three- 
phase  line  may  be  connected  Y  as  in  diagram  A,  Fig.  269,  or 


=  3Et 


cos  a 


o.ui; 


A    -    Y-  Connected      B-  Delta  Connected     C-  Delta  Connected 
Load  Load  Load 

.FiG.  269. — Connection  of  the  load  to  a  three-phase  circuit. 

delta  as  in  diagram  B.     If  the  lamps  shown  are  100- watt  lamps 
then,  when  Y-connected,  the  voltage  per  lamp  is  110/\/3  or  63.5 

volts  and  the  current  per  lamp  is  ~-_  =  1.57  amp.     When  delta 

Do.O 

1 00 
connected,  110- volt  lamps  are  required  and  they  take  :r~  =  0.91 


238     PRINCIPLES  OF  ELECTRICAL  ENGINEERING    [CHAP,  xxxi 

amp.  The  delta-connection  is  generally  used  for  the  connection 
of  individual  loads  across  the  phases,  the  distribution  circuits 
being  connected  to  the  power  mains  as  shown  in  diagram  C. 

If  one  of  the  lamps  in  diagram  B  burns  but,  the  two  remaining 
lamps  will  burn  with  their  normal  brilliancy,  but  if  one  of  the 
lamps  in  diagram  A  burns  out,  then  the  two  remaining  lamps  will 
be  in  series  across  110  volts  and  will  burn  dimly  since  they  are 
then  operating  at  55  volts  instead  of  at  the  normal  63.5  volts. 

267.  Power  Measurement  in  Polyphase  Circuits. — A  poly- 
phase circuit  is  one  with  more  than  one  phase.  To  measure  the 


3  Phase  Circuit 
'    2  Phase  Circuit 

FIG.  270.  FIG.  271. 

r   FIGS.  270  AND  271. — Wattmeter  connections  in  polyphase  circuits. 

power  in  a  two-phase  circuit,  each  phase  must  be  considered 
separately  and  two  wattmeters  used  as  shown  in  Fig.  270.  If 
the  load  is  balanced,  that  is  divided  equally  between  the  two 
phases,  then  only  one  set  of  instruments  is  required. 

If  in  a  balanced  two-phase  circuit 

E  =  100  volts 
I  =  50  amp. 
W  =  4000  watts 

then  the  total  power  =  4000  X  2  =  8000  watts 
the  apparent  power  =  (100  X  50)  X  2  =  10,000  volt  amperes 

the   power   factor  =  8000/10,000  =  0.8 

and  the  current  in  each  phase  lags  the  voltage  of  that  phase  by  an  angle  whose 
cosine  is  0.8  or  by  37  degrees. 

In  a  three-phase  circuit  it  is  usually  impossible  to  reach  the 
two  leads  of  each  phase  and  the  power  has  to  be  measured  out  on 
the  line  where  only  three  leads  are  available.  One  line,  for  ex- 
ample b,  Fig.  271,  is  supposed  to  be  the  return  line  for  the  other 
two  and  two  wattmeters  are  connected  as  shown  to  measure  the 
power  going  out  on  these  lines,  the  total  power  in  the  three-phase 
circuit  is  the  sum  of  the  readings  obtained  from  the  two  meters. 


ART.  268] 


ALTERNATORS 


239 


If  in  Fig.  271      E  =  100  volts 

/  =  50  amp. 
Wl  =  5000  watts 
TF2  =  2500  watts 

then  the  total  power  =  7500  watts 
the  apparent  power  =  1.73  X  100  X  50  =  8650  volt  amperes. 

the  power  factor  =  7500/8650  =  86.6  per  cent. 

and  the  current  in  each  phase  lags  the  voltage  of  that  phase  by  an  angle  whose 
cosine  is  0.866  or  by  30  degrees. 

268.  Alternator  Construction. — The  construction  of  a  revolving 
field  type  of  alternator  is  shown  in  Fig.  272. 


FIG.  272. — Revolving-field  type  of  alternator. 

The  stator  core  B  is  built  up  of  sheet  steel  laminations  which 
are  dovetailed  into  a  cast-iron  yoke  A  and  clamped  between  two 
iron  end  heads  E.  These  laminations  have  slots  C  on  their 
inner  periphery  and  in  these  slots  are  placed  the  armature  con- 
ductors D  which  are  insulated  from  the  slots  and  are  connected 
together  to  form  a  winding  from  which  e.m.f.  is  supplied  to  an 
external  circuit.  The  stator  core  is  divided  into  blocks  by  means 
of  vent  segments  F  and  the  ducts  thereby  provided  allow  air  to 
circulate  freely  through  the  machine  and  keep  it  cool. 


240     PRINCIPLES  OF  ELECTRICAL  ENGINEERING    [CHAP,  xxxi 

The  rotor  or  revolving  field  system  consists  of  a  series  of  N 
and  S-  poles  carrying  exciting  coils  H  and  mounted  on  an  iron 
field  ring.  An  alternator  has  to  be  excited  with  direct  current, 
it  cannot  therefore  be  self  exciting.  The  exciting  current,  gener- 
ally supplied  by  a  small  direct-current  generator  called  an  exciter, 
is  led  into  the  field  coils  through  brushes  M  which  bear  on  slip 
rings  insulated  from  the  shaft. 

The  exciter  voltage  is  independent  of  that  of  the  alternator  and 
is  generally  chosen  as  120  volts  so  that,  in  the  case  of  high- voltage 
alternators,  the  exciting  current  may  be  larger  than  the  full-load 
current  of  the  machine  as  in  the  following  case: 

A  single-phase  alternator  has  an  output  of  1000  kw.  at  13,200  volts  and 
100  per  cent,  power  factor,  find  the  current  at  full-load.  If  the  exciter  vol- 
tage is  120  and  the  excitation  loss  is  2  per  cent,  find  the  output  of  the  exciter 
and  also  the  exciting  current. 

a.  Watts  =  volts  X  amperes  X  power  factor, 
therefore  1000  X  1000  =  13,200  X  amperes  X  1.0 
and  amperes  at  full-load  =  76 

6.  The  exciter  output  =  2  per  cent,  of  1000  kw.  =  20  kw. 
20  X  1000 

The  exciting  current  =  •  -•-  ^n         =  167  amp. 

269.  The  revolving  armature  type  of  alternator  is  generally 
cheaper  than  the  revolving  field  type  of  machine  for  small  outputs 
at  low  voltages.  Such  a  machine  is  shown  diagrammatically  in 
Fig.  273;  the  armature  is  the  same  as  that  of  a  direct-current 
generator  except  that  the  commutator  is  removed  and  the  arma- 
ture is  tapped  at  two  diametrically  opposite  points  m  and  n  which 
are  connected  to  slip  rings  1  and  2. 

The  e.m.f.  between  these  slip  rings  is  a  maximum  when  the 
armature  is  in  the  position  shown,  and  is  zero  when  the  armature 
has  moved  through  quarter  of  a  revolution  from  this  position 
because  then  the  voltages  generated  in  the  conductors  between  m 
and  c  are  opposed  by  the  equal  voltages  in  the  conductors  between  c 
and  n.  The  e.m.f.  again  becomes  a  maximum  after  the  armature 
has  moved  through  half  of  a  revolution  from  the  position  shown  in 
Fig.  273  but  the  polarity  of  the  slip  rings  is  now  reversed.  The 
e.m.f.  between  the  slip  rings  is  therefore  alternating  and  goes 
through  one  cycle  per  pair  of  poles  passed. 

If  the  armature  is  tapped  at  four  points  as  shown  in  Fig.  274, 
the  voltage  EI  between  the  slip  rings  1  and  2  is  a  maximum  when 
the  armature  is  in  the  position  shown,  while  the  voltage  E2 
between  the  rings  3  and  4  is  zero  at  the  same  instant,  and  Ez 


ART.  270] 


ALTERNATORS 


241 


lags  EI  by  90  degrees  so  that  the  machine  is  now  a  two-phase 
alternator. 

To  obtain  three-phase  currents  the  armature  must  be  tapped 
at  three  points  as  shown  in  Fig.  275,  it  then  becomes  a  three 
phase  delta-connected  armature.  At  the  instant  shown,  the 


FIG.  273.— Single-phase.    FIG.  274.— Two-phase.    FIG.  275.— Three-phase. 
FIGS.  273-275. — Revolving  armature  type  of  alternator. 


FIG.  276. — Inductor  alternator. 

voltage  E2  is  zero  while  EI  is  positive  and  decreasing  and  Ez  is 
negative  and  increasing,  this  corresponds  to  instant  a  on  the 
voltage  curve  diagram. 

270.  The  inductor  alternator,  one  type  of  which  is  shown  dia- 
grammatically  in  Fig.  276,  has  been  found  suitable  for  the  gener- 
ic 


242     PRINCIPLES  OF  ELECTRICAL  ENGINEERING    [CHAP,  xxxi 

ation  of  high  frequency  e.m.fs.,  because  of  the  simplicity  of  the 
mechanical  construction. 

The  stationary  field  coil  F,  when  excited,  produces  a  magnetic 
flux  0  which  causes  all  the  inductors  N  to  have  the  same  polarity. 
The  coils  C  are  cut  by  the  lines  of  force  as  the  inductors  rotate, 
and  the  generated  voltage  is  a  maximum  when  the  inductors  are 
in  the  position  shown  and  one  side  of  each  coil  is  cutting  lines  of 
force,  the  voltage  is  zero  when  the  poles  are  in  position  y  and  is  a 
maximum  again  but  in  the  opposite  direction  when  the  poles 
are  in  the  position  z  and  the  other  side  of  each  coil  is  now  cutting 
the  lines  of  force.  The  voltage  therefore  passes  through  one- 
half  cycle  while  the  inductors  move  from  x  to  z  or  10  cycles  are 
passed  through  per  revolution,  so  that  a  machine  with  five  in- 


FIG.  277. — Magneto  alternator. 

ductors  is  equivalent  to  a  ten-pole  revolving  field  machine.  Since 
only  one  side  of  each  coil  is  active  at  any  instant  in  the  case  of  the 
inductor  alternator,  it  is  the  heavier  of  the  two  machines  for  a 
given  output  and  is  therefore  used  only  when  simplicity  of  con- 
struction is  essential. 

271.  Magneto  Alternators. — Two  types  of  alternating-current 
magnetos  used  for  gas-engine  ignition  are  shown  in  Figs.  277  and 
278.  In  the  former  machine,  the  armature  coil  C  is  stationary 
and  the  flux  threading  this  coil  is  varied  by  the  rotating  inductor 
ab,  whereas  in  the  latter  machine  the  coil  is  wound  on  the  inductor 
and  rotates  with  it. 

In  each  case,  when  the  inductor  is  in  the  position  shown,  the 
flux  0  passes  through  the  coil  C  from  a  to  6;  half  a  revolution  later 


ART.  271] 


ALTERNATORS 


243 


6  is  under  tho  N  pole  and  a  under  the  S  pole  and  the  flux  $  now 
passes  from  o  to  a  and  therefore  passes  through  coil  C  in  the  oppo- 
site direct  on. 

A  high  peak  of  e.m.f .  is  obtained  from  such  machines  by  shaping 
the  pole  faces  of  the  revolving  parts  so  that  the  flux  threading  the 
coil  C  changes  as  shown  in  curve  a,  Fig.  279;  the  flux  changing 
very  rapidly  as  the  inductor  moves  from  under  the  poles  into  the 
neutral  position.  The  e.m.f.  in  the  coil  C,  being  proportional  to 
the  rate  of  change  of  the  flux,  has  then  its  maximum  value  as  shown 


a -Flux 


FIG.    278. — Magneto    alter-    FIG.  279. — Voltage  and  current 
nator.  curves  in  a  magneto  alternator. 

in  curve  b,  Fig.  279.  The  current  lags  the  voltage  by  an  angle  a 
which  increases  as  the  reactance  of  the  circuit  increases  and  there- 
fore increases  with  the  frequency  of  the  alternator  or  the  speed  of 
the  engine. 


CHAPTER  XXXII 
ALTERNATOR  CHARACTERISTICS 


272.  Armature  Reaction. — Part  of  the  winding  of  an  alter- 
nator is  shown  in  Fig.  280,  the  poles  being  stationary  and  the 
field  coils  not  excited.  If  an  alternating  current  I  is  passed 
through  this  winding  from  an  external  source  then  lines  of  force 
will  encircle  the  coils  as  shown.  This  magnetic  field  is  alter- 
nating and  induces  in  the  coils  an  e.m.f.  of  self  induction  which 
opposes  the  applied  e.m.f.  and  is  equal  to  IX,  see  page  208,  where 


in 

FIG.  280.— Magnetic   flux  due  to       FIG.    281.— Dia-  FIG.    282.— Vec- 

the  armature  current.              grammatic     repre-  tor  diagram  for  an 

sentation  of  an  al-  alternator, 
ternator. 


/  is  the  current  flowing  and  X  is  the  reactance  of  the  winding  due 
to  its  self  induction.  An  alternator  may  therefore  be  considered 
as  a  circuit  with  a  resistance  R  and  a  reactance  X  as  shown 
diagrammatically  in  Fig.  281.  The  value  of  X  is  generally  from 
4  to  10  times  the  value  of  R. 

If  now  an  alternator  is  operating  under  normal  conditions,  fully 
excited,  generating  voltage  and  supplying  current  then,  of  the 
total  voltage  generated,  a  portion  IX  is  required  to  overcome  the 

244 


ART.  273] 


ALTERNATOR  CHARACTERISTICS 


245 


reactance  of  the  winding  and  another  portion  IR  to  overcome  the 
resistance;  the  terminal  voltage  Et  is  obtained  from  the  generated 
voltage  E0  by  subtracting  IR  and  IX  as  vectors. 

273.  Vector  Diagram  at  Full-load.— If  in  Fig.  282,  E0  is  the 
voltage  generated  by  an  alternator  at  no-load,  and  a  circuit  is  then 
connected  across  the  alternator  terminals  which  takes  a  current  / 
from  the  machine,  the  armature  resistance  drop  IR  is  in  phase 
with  the  current  while  the  current  lags  the  armature  reactance 
drop  IX  by  90  degrees,  see  page  207,  and  the  terminal  voltage 
Et  is  obtained  by  subtracting  IX  .and  IR  from  E0  as  shown  in 
Fig.  282. 

Three  cases  are  shown  in  Figs.  283,  284,  and  285,  in  which  an 
alternator  has  the  same  terminal  voltage  Et  and  delivers  the  same 


IX 


IX 


FIG.   283.— Lagging        FIG.  284.— 100  per      FIG..  285.— Leading 

current.  cent,  power  factor.  current. 

FIGS.  283-285. — Effect  of  the  power  factor  of  the  load  on  the  regulation  of 

an  alternator. 

current  /,  but  different  circuits  are  used  in  the  three  cases  so  that 
the  phase  angles  a  are  different. 

It  may  be  seen  from  these  diagrams  that  the  regulation  of  an 
alternator,  namely,  the  ratio  (E0  —  Et}/Et,  depends  largely  on 
the  power  factor  of  the  load,  and  becomes  negative  if  the  current 
is  leading  considerably,  as  shown  in  Fig.  285  where  Et  is  greater 
than  E0. 

274.  Regulation  Curves  of  an  Alternator. — Since  the  regulation 
of  an  alternator  depends  on  the  power  factor  of  the  load  as  well 
as  on  the  current,  the  external  characteristics  have  to  be  given 
with  different  power  factors  as  shown  in  Fig.  286.  These  curves 
are  generally  determined  by  calculation  after  the  resistance  and 
reactance  of  the  winding  have  been  measured. 


246    PRINCIPLES  OF  ELECTRICAL  ENGINEERING   [CHAP,  xxxn 


A  single-phase  alternator  with  an  output  of  416  amp.  at  2400  volts  has  a 
resistance  of  0.2  ohms  and  a  reactance  of  2.3  ohms.  Find  the  regulation  at 
100  per  cent,  power  factor,  and  also  at  80  per  cent,  power  factor  with  a  lag- 
ging current,  the  full-load  voltage  being  2400  volts  in  each  case. 

Full  -load  current  =   416  amp. 
the  resistance  drop  IR  =  416  X  0.2  =  83  volts 
the  reactance  drop  IX  =  416  X  2.3  =  960  volts. 
At  100  per  cent,  power  factor,  see  Fig.  284 

E<?  =  (Et  +  IR)2  +  (ixy- 

=  24832  +  9602 
and  Eo    =  2660 

the  regulation  =  —^—p*  —  =  2400  =  ^'^  per  cent- 


30  %  PoWer  Factor,  Leading 


Armature  Current 


FIG.  286.  —  Regulation  curves  of  an  alternator. 
At  80  per  cent,  power  factor  with  lagging  current,  see  Fig.  283 

Eo*  =  ab*  +  be2 

=  (Et  cos  a.  +  IRY  +  (Et'sin  a.  4-  IX)* 
=  (2400  X  0.8  +  83)2  4-  (2400  X  0.6  +  960)2 
and  Eo  =  3120 

.  ..  3120  -  2400 

the  regulation  =  -        ---  = 


275.  Experimental  Determination  of  Alternator  Reactance.  — 

The  no-load  saturation  curve  in  Fig.  287  is  determined  in  the 
same  way  as  for  a  direct-current  generator,  see  page  70.  The 
alternator  is  then  short-circuited  through  an  ammeter  as  shown 
in  diagram  B,  and  run  at  normal  speed,  while  simultaneous  read- 
ings are  taken  of  the  armature  current  Ia  and  the  exciting  cur- 
rent //  from  which  the  short-circuit  curve  is  plotted. 


ART.  275] 


ALTERNA TOR  CHARACTERISTICS 


247 


From  these  two  curves  the  reactance  of  the  alternator  may 
readily  be  determined.  With  an  exciting  current  oa  for  example, 
the  voltage  generated  by  the  alternator  is  ab  =  2400  volts  at  no- 
load.  With  the  same  excitation  and  with  the  armature  short- 
circuited,  the  terminal  voltage  is  zero  and  the  generated  voltage 
ab  is  used  up  in  sending  a  current  ac  =  1040  amp.  through  the 
resistance  and  reactance  of  the  winding  or 

voltage  ab  =  current  ac  X   VjR2  +  X2 

2400  =  1040  X   V^RMTY2 
and  V5r~^  =  2.3  ohms 


from  which  X  may  be  determined  if  the  value  of  R  is  known,  and 
this  may  readily  be  measured  by  passing  a  direct  current  I 
through  the  alternator  winding  and  measuring  the  voltage  drop 
E,  since  R  =  E/I. 


« 

«  o   3 

- 


5   v   F 

lie 


A  -  Connection  for  No  Load 
Saturation  Test 


B-  Connection  for  Short  Circuit  Test 

o  o 

Exciting  Current   If 

FIG.  287. — Determination   of   the   impedence   of   an   alternator. 

It  may  be  seen  from  Fig.  287  that  the  reactance  X  decreases 
as  the  magnetic  circuit  of  the  machine  becomes  saturated.  The 
reactance  is  caused  by  the  flux  produced  by  the  armature  current, 
as  shown  in  Fig.  280.  When  the  field  coils  are  excited  so  that 
the  main  flux  of  the  machine  is  passing  through  the  poles,  then 
a  smaller  additional  armature  flux  is  produced  with  a  given 
armature  current  than  when  the  poles  are  not  excited,  and  a 
smaller  armature  flux  produces  a  smaller  reactance. 

1.  A  three-phase  Y- connected  alternator  has  an  output  of  240  amp.,  at 
2400  volts.  With  a  certain  field  excitation  the  no-load  voltage  between 
terminals  was  2400  volts  and  the  current  in  each  line  on  short-circuit  was 
600  amp.  The  resistance  of  eacji  phase  is  0.2  ohms.  Find  the  reactance 
per  phase. 


248    PRINCIPLES  OF  ELECTRICAL  ENGINEERING    [CHAP,  xxxn 

Et,  the  terminal  voltage  at  no-load          =  2400  volts 

E,  the  voltage  per  phase  at  no-load          =  2400/  \/3,  page  236 

=  1390  volts 

Ii,  the  line  current  on  short-circuit         =  600  amp. 
7,  the  current  per  phase  on  short-circuit  =  600  amp.,  page  236 


Z,  the  impedence  per  phase  =  -----  =2.3  ohms 

600 

X,  the  reactance  per  phase  =  V  '~Z*  —  R2 

=  \/2.32  -  0.22 
=  2.3- 

Find  the  regulation  of  this  machine  at  full-load  and  100  per  cent,  power 
factor,  the  full-load  voltage  between  terminals  being  2400  volts. 

Full-load  current  in  line  =  240  amp. 

the  full-load  current  per  phase  =  240  amp. 

the  resistance  drop  IR  per  phase  =  240  X  0.2  =48  volts 

the  reactance  drop  IX  per  phase  =  240  X  2.3  =  550  volts 

the  full-load  voltage  between  terminals  =  2400  volts 

the  full-load  voltage  per  phase  =  2400/V3  =  1390  volts 

then  at  100  per  cent,  power  factor,  see  Fig.  284 

Eo2  =  (1390  +  48)2  +  5502 
and  Eo  =  1540  volts  per  phase 

and  the  no-load  voltage  between  terminals  =  1540  X  V'S 

=  2660  volts 
2660  -  2400 
the  regulation  =  ----  2400  --  =  per  cen*' 

2.  A  three-phase  delta-connected  alternator  has  an  output  of  240  amp.  at 
2400  volts.  With  a  particular  field  excitation  the  no-load  voltage  between 
terminals  was  2400  volts  and  the  current  in  each  line  on  short-circuit  was  600 
amp.  The  resistance  of  each  phase  was  0.6  ohms.  Find  the  reactance  per 
phase. 

Et,  the  terminal  voltage  at  no-load  =  2400  volts 

E,  the  voltage  per  phase  at  no-load  =  2400  volts,  page  237 

Ii,  the  line  current  on  short-circuit  =  600  amp. 

/,  the  current  per  phase  on  short-circuit  =  600/\/3,  page  237 

=  346  amp. 
2400 
Z,  the  impedence  per  phase  =    Q       =  6.9  ohms 


X,  the  reactance  per  phase  = 

=  6.9- 

Find  the  regulation  of  this  machine  at  full-load  and  100  per  cent,  power 
factor,  the  full-load  voltage  between  terminals  being  2400  volts. 

Full-load  current  in  the  line  =   240  amp. 
the  full-load  current  per  phase  =  240/\/3,  page  237 

=  139  amp. 


ART.  276] 


ALTERNATOR  CHARACTERISTICS 


249 


the  resistance  drop  IR  per  phase  =  139  X  0.6  =  83  volts 
the  reactance  drop  IX  per  phase  =  139  X  6.9  =  960  volts 
the  full-load  voltage  between  terminals  =  2400  volts 
the  full-load  voltage  per  phase  =  2400  volts,  page  237 
at  100  per  cent,  power  factor,  see  Fig.  284, 

Eo*  =  (2400  +  83  )2  +  9602 
Eo    =  2660  volts 
2660-2400 
the  regulation  =      "0400 —  =  per  cent- 

276.  Automatic  Regulators. — To  maintain  the  voltage  of  an 
alternator  constant,  the  field  excitation  must  be  increased  as  the 
armature  current  increases .  and  as  the  power  factor  decreases. 
This  cannot  be  done  by  adding  series  field  coils  as  in  the  case  of 
the  direct-current  generator,  see  page  74,  because  the  line  cur- 
rent is  alternating  and  not  suitable  for  excitation  purposes. 


r 

/-^WWv 
f°         /" 


Q 


M,CU 
*A 


FIG.  288. — Automatic  voltage-regulator. 

Automatic  regulators  are  used  with  alternators.  The  essen- 
tial parts  of  such  a  regulator  are  shown  diagrammatically  in  Fig. 
288.  To  keep  the  voltage  Et  constant,  some  means  must  be 
provided  to  close  the  contact  c  so  as  to  short-circuit  the  resistance 
r  and  thereby  increase  the  exciter  voltage  and  also  the  exciter 
current  ie  when  the  alternator  voltage  is  too  low,  and  to  open 
this  contact  and  insert  the  resistance  r  in  the  exciter  field  circuit 
when  the  alternator  voltage  is  too  high. 

The  contact  c  is  opened  by  the  electromagnet  M  on  which  are 
two  opposing  windings  A  and  B.  When  the  contact  a  is  open,  B 
alone  is  excited  and  opens  the  contact  c  against  the  tension  of  the 
spring  d,  but  when  a  is  closed,  the  coil  A  also  is  excited  and  neu- 
tralizes the  pull  of  B  and  the  spring  d  closes  the  contact  c. 

If  then  the  alternator  voltage  rises,  the  pull  of  the  solenoid  S  is 


250    PRINCIPLES  OF  ELECTRICAL  ENGINEERING    [CHAP,  xxxn 

increased  and  its  plunger  is  raised  so  as  to  open  the  contact  a, 
the  coil  A  is  then  deenergized  and  B  opens  the  contact  c  and 
inserts  the  resistance  r  in  the  exciter  field  circuit.  The  alternator 
voltage  now  drops,  the  pull  of  the  solenoid  S  decreases,  and  the 
weight  of  the  plunger  closes  the  contact  a  and  thereby  excites 
coil  A  which  neutralizes  the  pull  of  B,  the  spring  d  then  closes  the 
contact  c  and  short-circuits  the  resistance  r  and  thereby  increases 
the  field  excitation. 

When  additional  attachments  are  added  to  this  regulator  to 
make  it  more  sensitive,  the  voltage  fluctuations  on  which  the 
operation  of  the  regulator  depends  cannot  be  detected  on  a  sensi- 
tive voltmeter. 

277.  Efficiency. — The  losses  in  an  alternator  are  the  same  as  in 
a  direct- current  generator  and  consist  of  the  stray  loss,  the  arma- 
ture copper  loss,  and  the  field  excitation  loss.  These  losses  are 
determined  in  the  same  way  as  for  the  direct-current  machine,  see 
page  97.  It  must  be  noted  however  that  the  efficiency  depends 
on  the  power  factor  of  the  load,  as  may  be  seen  from  the  follow- 
ing example. 

In  a  two-phase  alternator  which  has  an  output  of  83  amp.  per  phase  at 
2400  volts 

the  stray  power  loss  =  16  kw. 

the  exciting  current  at  full-load  =  68  amp.  at  100  per  cent,  power  factor 

=  85  amp.  at  80  per  cent,  power  factor 

the  exciter  voltage  is  110  and  no  automatic  regulator  is  supplied 
the  resistance  of  each  phase  of  the  armature  winding,  measured  by 
direct  current  =0.4  ohms. 

Find  the  efficiency  when  the  power  factor  of  the  load  is  100  per  cent,  and 
also  when  80  per  cent.;  find  also  the  horse-power  of  the  driving  engine. 

at  100  per  cent,  power  factor  at  80  per  cent,  power  factor 

The  output  =  2  X  2400  X  83  =  2  X  2400  X  83  X  0.8 

=  400  kw.  =  320  kw. 

The  stray  loss                 =  16  kw.  =  16  kw. 
The  excitation  loss 

=  110  X  68              =  7.5  kw.  =  110  X  85      =  9.4  kw. 
The  armature  copper  loss 

=  (832  X  0.4)  X  2  =5.5  kw.  =  5.5  kw. 

The  total  loss                  =  29  kw.  =  30.9  kw. 

The  input                        =  429  kw.  =  350.9  kw. 
The  horse-power  of  the 

driving  engine           =  575  h.p.  =  470  h.p. 

400                                                         320 
The  efficiency  =  —         =  93.4  per  cent.  =  =91. 4  per  cent. 


ART.  278]  ALTERNATOR  CHARACTERISTICS  251 

The  lower  the  power  factor,  the  smaller  is  the  power  output,  and 
at  the  same  time  the  greater  the  excitation  loss  because  of  the 
increase  in  the  exciting  current  required  to  maintain  the  voltage. 

278.  Rating  of  Alternators.  —  An  alternator  is  designed  so  as  to 
give  normal  voltage  and  normal  current  without  overheating,  but 
the  output  in  kilowatts  will  depend  entirely  on  the  power  factor 
of  the  connected  load.  It  is  usual  to  specify  the  output  at  100 
per  cent,  power  factor  and  then,  to  emphasize  the  fact  that  this 
output  cannot  be  obtained  from  the  machine  at  lower  power  fac- 
tors, the  unit  of  output  is  taken  as  the  kilovolt  ampere  (kv.a.) 
and  not  as  the  kilowatt,  where  (kv.a.  X  power  factor)  =  kw. 

A  single-phase  alternator  can  give  100  amp.  at  2400  volts.  What  is  the 
output  of  the  machine  in  kv.a.  and  also  in  kw.  if  the  power  factor  of  the  load 
is  80  per  cent. 

2400  X  100 
kv.a.  =  ^~~=240 


kw.  =  240  X  0.8  =  192 

A  three-phase  alternator  can  give  100  amp.  from  each  terminal  with  a  vol- 
tage between  terminals  of  2400  then 

1.73  X  2400  X  100, 
the  output  in  kv.a.  =  —        —        —       "  see  page    ^  '  =      ^ 


at  80  per  cent,  power  factor  the  output  would   be  415  X  0.8  =  332  kw. 


CHAPTER  XXXIII 
SYNCHRONOUS  MOTORS  AND  PARALLEL  OPERATION 

279.   Principle  of    Operation    of   Synchronous   Motors. — An 

alternating- current  generator  may  be  made  to  operate  as  a  motor. 
When  an  alternating  e.m.f .  is  applied  to  the  winding  of  the  single- 
phase  machine  shown  in  Fig.  289,  an  alternating  current  flows 
through  that  winding.  The  conductors  a,  b,  c,  and  d  are  then 
carrying  current  and  are  in  a  magnetic  field  so  that  a  force  acts  on 
each  conductor,  while  an  equal  and  opposite  force  acts  on  the 
poles  and  tends  to  turn  the  rotor.  The  current  however  is 
alternating,  so  that  the  force  on  the  rotor  is  alternating  in  direc- 
tion unless  the  polarity  of  the  poles  is  changed  at  the  instant  the 
current  reverses. 


FIG.  289. — The  synchronous  motor. 

This  would  be  the  case  if  the  machine  was  already  running  at 
such  a  speed  that,  during  the  time  of  half  a  cycle  or  in  -^  seconds, 

the  rotor  moves  through  the  distance  between  two  adjacent  poles 
or  through  l/p  of  a  revolution.  This  speed,  called  the  synchron- 
ous speed,  is  therefore  equal  to 

X  2/rev.  per  sec. 

120/ 

or  rev.  per  mm. 

P 

252 


ART.  280] 


SYNCHRONOUS  MOTORS 


253 


and  is  the  speed  at  which  the  machine  would  have  to  run  as  an 
alternator  in  order  to  generate  an  e.m.f.  of/  cycles  per  second,  see 
the  formula  on  page  195.  The  table  on  page  195  therefore  applies 
to  synchronous  motors,  as  this  type  of  machine  is  called,  as  well 
as  to  alternators.  When  a  synchronous  motor  is  running  at 
synchronous  speed  it  is  said  to  be  in  step  with  the  alternators 
driving  it. 

A  synchronous  motor  is  not  self  starting,  but  will  develop  a 
torque  continuously  in  one  direction  if  running  at  synchronous 
speed.  Such  a  machine  is  generally  brought  up  to  speed  by  a 
small  self-starting  motor  which  is  direct  connected  to  the  shaft. 

280.  The  Back  E.m.f.  of  a  Synchronous  Motor. — If  the  motor 
M,  Fig.  290,  is  rotating  at  synchronous  speed,  then  its  stator  wind- 


Diagrammatic  Kcprcscutation 


jtor  Synchronous  Motors 

A      No  Load  D     Full  Load 

FIG.  290. — Alternator  driving  synchronous  motors. 


ing  is  being  cut  by  lines  of  force  and  an  e.m.f.  is  generated  in  the 
machine  in  the  same  way  as  if  it  were  driven  by  an  engine.  This 
e.m.f.  Em,  called  the  back  e.m.f.  of  the  motor,  opposes  the  applied 
e.m.f.  Eg  and  is  of  the  same  frequency,  and,  in  the  case  where  the 
two  machines  are  equally  excited,  then  Em  =  Eg. 

If  the  motor  is  running  on  no-load,  the  load  current  that  it  takes 
from  the  line  is  practically  zero,  being  merely  sufficient  to  over- 
come the  friction  of  the  machine,  and  in  such  a  case  Em  is  exactly 
equal  and  opposite  to  Eg  at  every  instant  as  shown  by  the  vectors 
in  Fig.  292,  for,  under  these  conditions,  there  is  no  resultant  e.m.f. 
and  no  flow  of  current  through  the  machines.  The  poles  of  the 


254    PRINCIPLES  OF  ELECTRICAL  ENGINEERING    [CHAP,  xxxn 

two  machines  will  then  rotate  together  with  the  relative  position 
shown  in  diagram  A,  Fig.  290. 

If  now  the  motor  is  loaded,  it  will  slow  down  for  an  instant 
and  will  swing  back  relative  to  machine  G  as  shown  in  diagram  B, 
Fig.  290.  The  back  e.m.f.  Emj  although  still  equal  to  Eg  is  now 
no  longer  opposite  in  phase  as  at  no-load,  but  lags  its  no-load 
value  by  an  angle  6  as  shown  by  the  vectors  in  Fig.  292.  There 
is  now  a  resultant  e.m.f.  Er  which  sends  a  current  through  the 
machines,  and  the  torque  developed  due  to  this  current  keeps 
the  motor  running  at  synchronous  speed  but  always  with  such  a 
lag  behind  the  generator  as  to  allow  a  current  to  flow  large  enough 
to  develop  a  driving  torque  equal  to  the  retarding  torque  of  the 
load.  The  motor  therefore  automatically  takes  from  the  genera- 
tor a  current  corresponding  to  the  mechanical  load. 


FIG.  291. — Mechanical  anal- 
ogy to  a  synchronous  motor. 


Em        Em 
No  load.     Full  load. 
FIG.  292. — Voltage  vector  dia- 
grams for  a  synchronous  motor. 


281.  Mechanical   Analogy. — The  transmission   of  power   by 
means  of  an  alternator  and  a  synchronous  motor  is  similar  in 
many  ways  to  the  transmission  of  power  by  means  of  a  flexible 
spring  coupling  such  as  that  shown  in  Fig.  291.     If  the  load  on 
the  side  M  is  increased,  the  spring  stretches  and  M  drops  back 
through  a  small  angle  relative  to  G,  but  both  continue  thereafter 
to  rotate  at  normal  speed. 

282.  Vector  Diagram  for  a  Synchronous  Motor. — In  Fig.  293 
Eg  is  the  e.m.f.  generated  in  the  alternator  winding. 

Em  is  the  back  e.m.f.  generated  in  the  motor  winding. 
Ert  the  resultant  e.m.f.,  sends  an  alternating  current  1  through 
both  machines. 


ART.  283] 


SYNCHRONOUS  MOTORS 


255 


Rm  and  Xm  are  the  resistance  and  reactance  of  the  motor  winding 
Rg  and  Xg  are  the  resistance  and  reactance  of  the  generator 
winding. 

Er 


and  since  the  resistances  are  generally  small  compared  with  the 
reactances,  see  page  244,  therefore 


xm+xg 

Since  the  circuit  is  almost  entirely  inductive,  the  resistance  being 
negligible,  the  current  /  must  lag  the  voltage  Er  by  90  degrees. 
The  power  developed  by  the  generator  =  Egl  cos  a. 


Em 

FIG.  293.  FIG.  294.  FIG.  295. — Load  greater  than 

Light  load.  Heavy  load.  the  maximum  output. 

FIGS.  293-295. — Vector  diagram  for  the  synchronous  motor  at  various  loads. 

If  the  load  on  the  motor  is  now  increased,  the  motor  swings 
back  relative  to  the  generator  by  a  greater  angle  6,  as  shown  in 
Fig.  294.  The  voltage  Er  and  the  current  /  are  now  larger  than 
before  and  so  also  is  Egl  cos  a  the  power  developed  by  the 
generator  and  put  into  the  circuit. 

283.  Maximum  Output. — An  extreme  case  is  shown  in  Fig.  295 
where  the  load  has  caused  the  motor  to  swing  back  by  a  large 
angle  6,  but  the  power  put  into  the  line,  namely  Egl  cos  a,  is 


256    PRINCIPLES  OF  ELECTRICAL  ENGINEERING  [CHAP,  xxxm 


now  less  than  in  the  case  represented  in  Fig.  294,  so  that,  while 
the  angle  6  increases  as  the  load  is  increased,  the  power  input 
Egl  cos  a  increases  only  up  to  a  certain  point  called  the  break- 
down point  or  point  of  maximum  output.  -  When  the  load  exceeds 
this  value  the  motor  slows  down  and  stops. 

The  maximum  output  is  generally  more  than  twice  the  normal 
output  of  the  machine  as  fixed  by  the  heating  of  the  windings. 

284.  Operation  of  a  Synchronous  Motor  when  Under-  and 
Overexcited. — In  diagram  B,  Fig.  296,  the  excitation  of  the 
motor  M  is  such  that  Em  =  Eg.  Two  other  conditions  of  opera- 
tion are  represented  in  diagrams  A  and  C.  In  the  former  case 
the  motor  is  overexcited  and,  since  the  speed  cannot  change 
but  must  always  be  synchronous  speed,  the  voltage  Em  must 

Eg  AEg  AEff 


'  7cos  or 


ABC 
FIG.  296. — Effect  of  excitation  on  the  power  factor  of  a  synchronous  motor. 

increase  with  the  excitation  and  must  now  be  greater  than  Eg. 
In  the  latter  case  the  motor  is  underexcited  and  Em  must  be  less 
than  Eg.  The  load  on  the  motor  however  is  unchanged  so  that 
Egl  cos  a  is  constant. 

It  is  important  to  note  that,  in  the  case  of  the  overexcited 
motor,  diagram  A,  the  current  /  leads  the  generator  voltage  Eg, 
or  an  overexcited  synchronous  motor  draws  a  leading  current 
from  the  line.  Now  a  condenser  always  draws  a  leading  current, 
see  page  220,  so  that  an  overexcited  synchronous  motor  acts  to  a 
certain  extent  like  a  condenser. 

285.  Use  of  the  Synchronous  Motor  for  Power  Factor  Correc- 
tion.— If  the  load  connected  to  an  alternator  has  a  low  power  fac- 
tor, it  is  often  advisable  to  arrange  that  some  of  the  load  shnll  l><> 


ART.  285] 


SYNCHRONOUS  MOTORS 


257 


carried  by  synchronous  motors  so  as  to  improve  the  power  factor 
of  the  whole  system. 

If  1000  horse-power  of  2200-volt  single-phase  induction  motors  are 
operating  at  the  end  of  a  transmission  line,  find  the  current  in  the  line  and 
also  the  generator  capacity  required  if  the  average  power  factor  is  80  per 
cent,  and  the  average  efficiency  is  90  per  cent.  (The  induction  motor,  see 
Chap.  36,  takes  a  lagging  current,  and  its  power  factor  cannot  be  controlled.) 

The  output  of  the  motors  is  1000  h.p. 

1 000 
the  input  to  the  motors  is  7Tq~  =  1115  h.p. 

=  830  kw. 

oorv 

the  generator  capacity  required  =  ~  ^  =  1040  kv.a. 

1040  X  1000 
the  current  in  the  line  =  ~onn      -  =  473  amp. 

ZZ(J(J 


FIG.  297. 


If  500  horse-power  of  the  load  is  driven  by  a  synchronous  motor,  the  power 
factor  of  the  whole  system  may  be  raised  if  this  motor  is  overexcited  and  made 
to  act  as  a  condenser. 

If  the  power  factor  of  the  synchronous  motor  be  made  80  per  cent.,  with 
the  current  leading,  then  the  vector  diagram  for  the  load  is  as  shown  in 
diagram  B,  Fig.  297. 

The  induction  motor  output  =  500  h.p. 

500        746 
the  induction  motor  input  =  -^g  X  JQQQ  =  415  kw. 

the  current  for  these  motors  =  ^onO  X  0  8   =  "^  amp. 

The  current  for  the  synchronous  motor  also  is  236  amp.,  but  it  leads  by  an 
angle  whose  cosine  is  0.8  whereas  the  current  for  the  induction  motor  lags  by 
the  same  angle 

the  resultant  current  in  the  line  =  2  (236  X  0.8) 

=  378  amp. 

378  X  2200 
the  generator  capacity  required  =       — JJQQQ 

=  830  kv.a. 

17 


258    PRINCIPLES  OF  ELECTRICAL  ENGINEERING  [CHAP,  xxxm 

By  the  use  of  an  overexcited  synchronous  motor,  the  power  fac- 
tor of  the  system  is  improved,  the  generator  capacity  required 
for  the  load  is  reduced,  and  so  also  is  the  current  in  the  line,  so  that 
overexcited  synchronous- motors  may  be -used  with  advantage  at 
the  end  of  a  long  line. 

286.  Synchronizing. — Before  a  synchronous  motor  can  carry 
a  load  it  must  be  started  and  brought  up  to  synchronous  speed, 
and  the  switch  S,  Fig.  298,  should  not  be  closed  until  Em  is  equal 
and  opposite  to  Eg.  If  this  switch  were  closed  when  Em  and  Eg 
are  acting  in  the  same  direction  round  the  closed  circuit,  then  the 
resultant  voltage  Er  would  be  equal  to  Em  +  Eg  and  would  send 
a  destructive  current  /  through  the  circuit.  When  Em  and  Ea 
are  equal  and  opposite,  then  Er  is  zero,  and  no  current  I  will 
flow  when  the  switch  S  is  closed.  To  test  for  this  condition,  a 
lamp  L,  or  an  indicating  device  called  a  synchroscope,  is  placed 


FIG.    298. — Alternator    driving    FIG.   299. — Alternators  in  par- 
a  synchronous  motor.  allel. 

across  the  switch  S.  When  the  lamp  is  brightest,  then  Er  = 
Em  -f-  Eg]  when  darkest,  then  Er  =  zero  and  the  switch  S  may 
be  closed,  after  which  the  load  may  be  put  on  the  motor.  The 
operation  described  above  is  called  synchronizing. 

287.  Hunting. — In  the  case  of  an  engine-driven  alternator,  and 
particularly  if  the  engine  is  a  gas  engine,  the  angular  velocity  is  not 
uniform  but  consists  of  a  uniform  angular  velocity  with  a  super- 
imposed oscillation,  the  frequency  of  the  generated  e.m.f.  there- 
fore is  not  constant,  but  rises  and  falls  regularly. 

If  this  e.m.f.  is  applied  to  a  synchronous  motor,  the  synchron- 
ous speed  of  the  motor  tends  to  rise  and  fall  regularly  with  the 
frequency,  and  the  motor  tends  to  have  a  superimposed  oscilla- 
tion similar  to  that  of  the  alternator.  If  the  natural  period  of 
oscillation  of  the  motor  has  the  same  frequency  as  this  forced 
oscillation  then  the  effect  will  be  cumulative  and  the  motor  will 
oscillate  considerably. 


ART.  288]  SYNCHRONOUS  MOTORS  259 

A  similar  result  would  be  found  with  the  model  shown  in  Fig. 
291.  If  the  torque  applied  .to  G  is  not  uniform,  then  G  will 
oscillate  about  its  position  of  mean  angular  velocity  and  M  will 
have  an  oscillating  force  impressed  on  it  by  the  spring.  If  the 
moment  of  inertia  of  the  flywheel  M  is  such  that  its  natural  fre- 
quency of  oscillation  is  the  same  as  the  frequency  of  the  impressed 
oscillation  then  (rand  M  will  swing  backward  and  forward  relative 
to  one  another  through  a  considerable  angle. 

As  the  two  machines  M  and  G  oscillate  relative  to  one  another, 
the  angle  6,  Fig.  294,  increases  and  decreases  regularly,  and  the 
value  of  both  Er  and  of  the  current  /  vary  above  and  below  the 
average  value  required  for  the  load.  This  surging  of  current  is  of 
comparatively  low  frequency  and  is  indicated  by  an  ammeter 
placed  in  the  circuit.  Due  to  this  surging,  the  circuit  breakers 
protecting  the  machines  may  be  opened  although  the  load  is  not 
greater  than  normal,  while  the  cumulative  swinging  of  the  ma- 
chines relative  to  one  another,  called  hunting,  may  cause  the 
motor  to  drop  out  of  step. 

To  prevent  hunting,  the  impressed  oscillations  must  be  elim- 
inated or  the  natural  frequency  of  vibration  of  the  motor  must 
be  changed.  The  methods  used  in  practice  to  minimize  hunt- 
ing are: 

1.  Dampen  the  governor  if  the  impressed  oscillations  are  found 
to  be  caused  by  a  hunting  governor. 

2.  Change  the  natural  period  of  vibration  of  the  machine  by 
changing  the  flywheel;  the  larger  the  moment  of  inertia  of  the 
rotating  part  of  the  motor,  the  longer  is  its  natural  period  of 
vibration. 

3.  Dampen  the  oscillations  electrically  by  the  use  of  pole 
dampers  such  as  those  described  on  page  295. 

288.  Parallel  Operation  of  Alternators. — Two  alternators 
connected  to  operate  in  parallel  are  shown  in  Fig.  299.  If  the 
voltage  of  machine  B  is  not  exactly  equal  and  opposite  to  that  of 
A  at  every  instant  then  current  will  flow  in  the  local  circuit 
between  the  two  machines  just  as  in  Fig.  298.  To  prevent  this 
the  two  machines  must  be  synchronized  in  the  same  way  as  an 
alternator  and  a  synchronous  motor;  when  the  lamp  L  in  Fig.  299 
is  dark,  then  Eb  is  exactly  equal  and  opposite  to  Ea  and  the 
machines  have  the  same  frequency;  the  switch  S  may  then  be 
closed.  If  now  the  engine  of  B  fails  for  an  instant,  then  generator 
B  will  tend  to  slow  down  and  will  swing  back  relative  to  A,  so  that 


260    PRINCIPLES  OF  ELECTRICAL  ENGINEERING  [CHAP,  xxxm 

current  will  flow  in  the  local  circuit  and  B  will  be  driven  as  a  syn- 
chronous motor  in  the  same  direction  as  before  and  at  the  same 
speed.  As  soon  as  the  engine  of  B  recovers,  the  generator  will 
swing  forward  again  and  carry  its  share -of  the  load. 

If  two  direct-current  generators  are  operating  in  parallel,  and 
the  field  excitation  of  one  of  the  machines  is  increased,  then  the 
voltage  of  that  machine  will  be  raised  and  it  will  take  a  larger 
portion  of  the  load.  An  increase  in  load  makes  the  engine  and 
generator  slow  down  and  allows  the  engine  to  draw  the  additional 
amount  of  steam  required  for  the  additional  load. 

If  two  alternators  are  operating  in  parallel,  an  increase  in  the 
excitation  of  one  machine  raises  the  voltage  of  that  machine  and 
tends  to  make  it  carry  more  of  the  load.  But  the  machine  can- 
not slow  down  and  allow  the  engine  to  take  more  steam  because  it 
can  run  only  at  synchronous  speed.  An  increase  in  excitation  of 
one  machine  therefore  increases  its  voltage  and  at  the  same  time 
makes  the  current  in  that  machine  lag  further  behind  the  voltage 
so  as  to  maintain  the  load  El  cos  a  constant  at  the  value  corre- 
sponding to  the  steam  supply.  To  change  the  distribution  of 
load  between  two  alternators  operating  in  parallel,  the  governors 
of  the  driving  engines  must  be  manipulated  so  as  to  change  the 
distribution  of  the  steam  supply. 

As  the  total  load  on  the  two  alternators  increases,  they  both 
slow  down,  and  the  engine  governors  automatically  allow  the 
necessary  amount  of  steam  to  flow,  while  the  frequency  of  the 
generated  e.m.f.  decreases  slightly.  In  order  that  the  two 
machines  may  divide  the  load  properly,  the  engines  should  have 
the  same  per  cent,  drop  in  speed  between  no-load  and  full-load. 

The  same  applies  to  alternators  driven  by  water  wheels.  To 
make  any  one  of  a  number  of  turbine-driven  alternators  take  a 
larger  portion  of  the  total  load,  the  governor  of  that  machine 
must  be  manipulated  to  allow  the  turbine  to  take  more  water. 


CHAPTER  XXXIV 


TRANSFORMER  CHARACTERISTICS 

289.  In    order    that    electric    energy    may   be    transmitted 
economically  over  long  distances,  high  voltages  must  be  used; 
but  in  order  that  electric  circuits  may  be  safely  handled,  low  vol- 
tages are  necessary  for  distribution.     The    alternating-current 
transformer  is  a  piece  of  apparatus  by  means  of  which  electricity 
can  be  received  at  one  voltage  and  delivered  at  another  voltage 
either  higher  or  lower.     It  consists  essentially  of  two  coils  wound 
on  an  iron  core;  one  coil  receives  energy  and  is  called  the  primary 
coil,  the  other  delivers  energy  and  is  called  the  secondary  coil. 

290.  Constant  Potential  Transformer.— In  Fig.   300,   C  is  a 
closed  magnetic   circuit  on  which 

are  wound  two  coils  having  n\  and 
n<i  turns  respectively.  When  an 
alternating  e.m.f.  e\  is  applied  to 
the  coil  n\  while  coil  HI  is  closed 
through  a  circuit  as  shown,  then  a 
current  i\  flows  in  the  primary  coil 
and  produces  an  alternating  mag- 
netic flux  $  which  threads  both 

coils  and  generates  in  them  electromotive  forces  e\b  and  62  which 
are  proportional  to  n\  and  n^  the  number  of  turns. 

Now  eib  is  called  the  back  e.m.f.  of  the  primary  and,  according 
to  Lenz's  law,  opposes  the  change  of  the  flux  which  produces  it 
and  therefore  opposes  e\  which  produces  the  change  of  flux;  e\b 
is  less  than  e\  by  the  e.m.f.  required  to  send  the  current  i\  through 
the  primary  coil,  which  e.m.f.  is  small  in  modern  transformers  and 
seldom  exceeds  1  per  cent,  of  'e\  even  at  full-load. 

The  e.m.f.  e2  sends  a  current  &Y  through  the  secondary  winding 
in  such  a  direction  as  to  oppose  the  change  of  the  flux  4>  which 
produces  it,  and  therefore  to  oppose  i\\  but  the  magnetizing 
effect  of  ii  must  always  be  greater  than  the  demagnetizing  effect 
of  iz  by  the  amount  necessary  to  produce  the  flux  4>  in  the  mag- 
netic circuit. 

261 


ecoi.dary 


FIG.  300. — The  transformer. 


262    PRINCIPLES  OF  ELECTRICAL  ENGINEERING    [CHAP,  xxxiv 

If  now  the  impedence  of  the  secondary  circuit  is  decreased,  the 
current  i2  will  increase  and  thereby  reduce  the  flux  4>.  But  a 
reduction  in  <£  causes  the  back  e.m.f.  e\b  to  decrease  and  thereby 
allow  a  larger  current  i\  to  flow  in  the  primary  winding,  so  that 
the  primary  current  always  adjusts  itself  to  suit  the  requirements 
of  the  secondary  circuit. 

Now  the  primary  resistance  is  so  small  that  eu>  and  <£  do  not 
drop  more  than  about  1  per  cent,  between  no-load  and  full-load, 
so  the  statement  may  be  made  that  <£  is  constant  at  all  loads  and 
therefore  the  resultant  of  n\i\  and  n2?'2,  the  primary  and  the 
secondary  ampere-turns,  must  always  be  equal  to  some  quantity 
niio  which  produces  the  constant  flux  </>. 

The  quantity  n\iQ  may  readily  be  found  because,  if  the  second- 
ary circuit  be  opened,  no  current  can  flow  in  that  winding,  and 
the  current  in  the  primary  under  these  conditions  has  merely  to 
produce  the  flux  </>.  This  current  must  therefore  be  IQ  and  is 
called  the  magnetizing  current  of  the  transformer. 

Since  e\b  and  e2  are  produced  by  the  same  magnetic  flux  <f>  they 
are  proportional  to  the  number  of  turns  HI  and  nz,  and  since  e\b  is 
practically  equal  to  e\  therefore 


and  is  called  the  ratio  of  transformation. 

It  has  also  been  shown  that  the  resultant  of  n\i\  and  of 
must  always  be  equal  to  n^'o  where  io,  called  the  magnetizing 
current,  is  comparatively  small,  so  that  if  ^o  be  neglected  then 


ni      12      e\ 

Or  —  as  -r-  SB.  - 

nz      ^l      ez 
and  e\i\  =  e&z 

According  to  the  law  of  conservation  of  energy,  the  input  to  a 
transformer  must  be  exactly  equal  to  the  output,  the  losses  being 
neglected  (the  efficiency  of  a  50-kilovolt  ampere  transformer  is 
98  per  cent.)  therefore 

e\i\  cos  CL\  =  62^2  cos  a  2 
and  e\i\  =  62^2 

therefore  cos  <*i  =  cos  0:2 

so  that,  neglecting  the  magnetizing  current-  and  the  losses,  the 
power  factor  of  the  primary  is  the  same  as  that  of  the  secondary. 


ART.  292]  TRANSFORMER  CHARACTERISTICS  263 

291.  Vector  Diagram  for  a  Transformer. — (a)  No-load  condi- 
tions. For  a  transformer  which  has  a  negligible  no-load  loss,  the 
voltage  and  current  phase  relations  are  shown  in  Fig.  301. 

</>  is  the  magnetic  flux  threading  both  coils. 

z'o  is  the  magnetizing  current  which  produces  4>. 

e2  and  db  are  the  e.m.fs.  generated  in  the  coils  nz  and  HI 
respectively,  and  lag  the  flux  which  produces  them  by  90  degrees, 
see  the  footnote  on  page  207. 

ei,  the  applied  primary  e.m.f.,  is  equal  and  opposite  to  eib. 

(b)  Full-load  conditions.  Let  the  secondary  circuit  now  be 
closed  and  let  its  resistance  and  reactance  be  such  that  iz  lags 
ez  by'  a2  degrees,  then  the  voltage  and  current  phase  relations  are 
as  shown  in  Fig.  302. 

0  is  the  magnetic  flux  threading  both  coils  and  has  practically 
the  same  value  at  full-load  as  at  no-load. 


to 


FIG.  301.— No  load.  FIG.  302.— Full  load. 

Vector  diagrams  for  a  transformer. 

10  is  the  component  of  the  primary  current  required  to  produce 
the  flux  0. 

e\  is  the  applied  primary  e.m.f. 

ez  is  the  secondary  generated  e.m.f. 

iz  is  the  secondary  current,  whose  value  and  whose  phase  angle 
&2  depend  on  the  constants  of  the  connected  circuit. 

in  is  the  component  of  the  primary  current  required  to  neu- 
tralize the  demagnetizing  effect  of  the  current  &2; 

niin  =  n2i2 

11  is  the  primary  current  and  is  the  resultant  of  z'o  and  in. 
If  the  current  it  is  small,  then  ai  =  a2  and  nii\  =  n&z. 

292.  Induction  Furnace. — An  electric  furnace  which  operates 
as  a  transformer  is  shown  diagrammatically  in  Fig.  303  and  is 
called  an  induction  furnace,  the  secondary  winding  in  this  case 
being  the  charge  which  is  contained  in  the  annular  channel  A 


264    PRINCIPLES  OF  ELECTRICAL  ENGINEERING    [CHAP,  xxxiv 

and  is  heated  by  the  secondary  current.  The  amount  of  energy 
put  into  the  secondary  can  be  varied  by  varying  the  applied 
primary  voltage. 

Fig.  304  shows  diagrammatically  an  electric  welder  which 
operates  on  the  same  principle.  The  single  turn  A  in  this  case 
is  open,  and  is  closed  by  the  two  pieces  to  be  welded,  which 
pieces  are  held  in  the  clamps  B  and  are  forced  together  under 
pressure  while  the  welding  current  passes  across  the  contact  and 
heats  the  ends  to  be  joined. 

It  might  seem  that,  since  the  secondary  load  in  this  case  is  a 
resistance  load  being  the  resistance  of  the  molten  metal,  the 
power  factor  of  the  transformer  would  be  high  at  full-load,  and 
yet  in  practice  it  seldom  exceeds  70  per  cent.  To  explain  the 


V^Primary 
Winding 


FIG.  303. — Induction  furnace.         FIG.  304. — Induction  welder. 


cause  of  this  low  power  factor  it  is  necessary  to  take  up  the  subject 
of  leakage  flux  in  transformers. 

293.  Leakage  Reactance. — Fig.  306  shows  the  actual  flux 
distribution  in  a  transformer.  The  ampere-turns  n\i\  produce 
a  flux  <t>u,  called  the  primary  leakage  flux,  which  is  proportional 
to  ii  and  which  threads  the  coil  n\  but  does  not  thread  n^. 

The  ampere-turns  n2iz  produce  a  flux  fai,  called  the  secondary 
leakage  flux,  which  is  proportional  to  i%  and  which  threads  the 
coil  HI  but  does  not  thread  n\. 

The  ampere-turns  niii  and  n^iz  acting  together  produce  a 
magnetic  flux  </>  which  threads  both  coils  fti  and  n2  and  which  is 
practically  constant  in  magnitude;  the  effect  of  this  constant  flux 
has  already  been  considered. 

Now  any  coil  in  which  a  current  i  produces  a  flux  <j>  which  is 
proportional  to  the  current  is  said  to  have  self  induction,  see 


ART.  293]  TRANSFORMER  CHARACTERISTICS 


265 


Art.  239,  page  206,  and  the  voltage  to  send  an  alternating  cur- 
rent /  through  such  a  coil  =  IX  where  X  is  the  reactance  of 
the  coil;  the  current  lags  this  voltage  by  90  degrees,  seepage 
208. 

In  Fig.  306,  the  flux  <f>u  is  proportional  to  the  current  it  and 
its  effect  is  the  same  as  if  the  coil  n\  had  a  reactance  Xi  so  that, 
instead  of  considering  the  effect  of  the  flux  <£H,  the  effect  of  the 


7 


FIG.  305.  FIG.  306.  FIG.  307. 

FIG.  305. — Ideal  transformer. 

FIG.  306. — Actual  flux  distribution  in  a  transformer. 
FIG.  307. — Transformer  showing  the  resistances  and  reactances  diagram- 
matically. 

equivalent  reactance  Xi  may  be  considered.  In  the  same  way 
the  leakage  flux  fai  may  be  represented  by  an  equivalent  reactr 
ance  X2.  Fig.  307  shows  the  diagram  of  an  actual  transformer 
in  which  the  leakage  fluxes  <f>u  and  fai  are  replaced  by  the  equiva- 
lent reactances  Xi  and  X2  which,  along  with  the  resistances  Ri 
and  R2  of  the  coils,  are  placed  for  convenience  outside  of  the  actual 
winding. 


FIG.    308. — Vector    diagram 
of  an  ideal  transformer. 


FIG.    309. — Vector   diagram 
of  an  actual  transformer. 


Between  the  terminals  ab  and  cd,  the  transformer  diagram  in 
Fig.  307  is  the  same  as  the  ideal  diagram  in  Fig.  305.  The  vector 
diagram  for  this  ideal  transformer  is  shown  in  Fig.  308  which  is 
the  same  as  Fig.  302,  page  263. 

The  actual  terminal  voltage  Et  is  obtained  by  subtracting  from 
EZ  the  vectors  I2R2  and  I2X2,  the  voltages  to  overcome  the  sec- 


266    PRINCIPLES  OF  ELECTRICAL  ENGINEERING  [CHAP,  xxxiv 

ondary  resistance  and  reactance  respectively,  where  I^Rz  is  in 
phase  with  72  and  72  lags  IiX2  by  90  degrees. 

The  applied  primary  voltage  Ea  is  obtained  by  adding  to  Ei 
the  vectors  I\R\  and  /iXi,  the  primary  resistance  and  reactance 
drops. 

In  the  elementary  discussion  of  transformer  operation,  the 
resistance  and  reactance  drops  were  neglected  and  it  was  found 

TjJ 

that  —  -  =  —    In  Fig.  309  this  relation  still  holds,  but  the  ratio 
LI       n\ 

-  is  less  than  ;-  and*  so  less  than      ,  so  that  the  transformer 

Ea  El  Hi 

ratio  decreases  as  the  load  increases  or  the  secondary  voltage 
drops  with  increase  of  load;  this  drop  however  is  small,  being 
less  than  2  per  cent,  for  a  5  kv.a.  transformer  at  100  per  cent, 
power  factor,  while  for  leading  currents  in  the  secondary  circuit 
the  secondary  voltage  may  rise  with  increase  of  load  just  as  in 
the  case  of  the  alternator,  see  page  245;  the  student  may  satisfy 
himself  on  this  point  by  drawing  the  vector  diagram. 

294.  Leakage  Reactance  in  Standard  Transformers  and  in  In- 
duction Furnaces. — In  power  transformers,  the  leakage  reactances 


Primary 


Primary 


Secondary 


A  B 

FIG.  310. — Leakage  flux  of  transformers. 

XL  and  X$  are  kept  small  by  constructing  the  transformer  so 
that  0u  and  fai  are  small.  Diagram  A,  Fig.  310,  shows  a  trans- 
former with  a  primary  and  a  secondary  coil  on  each  leg  and  shows 
also  the  leakage  fluxes.  It  may  be  seen  from  this  diagram  that, 
on  each  leg,  0u  and  fai  act  in  opposite  directions,  so  that  if  n2 
were  interwound  with  n\  then  the  leakage  fluxes  would  neutralize 
and  only  the  main  flux  0  be  left.  This  result  is  approximated  in 
practice  by  constructing  the  transformer  as  shown  in  diagram  B, 
where  half  of  the  primary  and  half  of  the  secondary  winding  are 
placed  over  one  another  on  each  leg  of  the  transformer  core,  the 
leakage  fluxes  have  then  to  crowd  into  the  space  x  between  the 


ART.  295]  TRANSFORMER  CHARACTERISTICS  207 

windings,  and  the  smaller  the  space  x,  the  smaller  the  leakage 
fluxes  and  the  smaller  the  leakage  reactances. 

In  the  induction  furnace,  unfortunately,  the  distance  x  cannot 
be  made  small,  so  that  the  reactances  are  comparatively  large. 
The  vector  diagram  for  such  a  furnace  is  shown  in  Fig.  311.  The 
secondary  winding  is  short-circuited  since  it  consists  of  a  ring  of 
molten  metal,  so  that  Et,  the  terminal  voltage,  is  zero,  and  the 
secondary  generated  voltage  Ez  is  therefore  made  up  of  the  two 
components  I2Rz  and  /2^f  2,  where  R%  is  the  resistance  of  the  ring 
of  molten  metal  and  X%  its  reactance  due  to  the  leakage  flux  <£2z, 
Fig.  307.  The  remainder  of  this  diagram  is  determined  in  the 
same  way  as  in  Fig.  309,  and  the  power  factor  of  the  furnace  is 
the  cos  ai  and  seldom  exceeds  70  per  cent. 


FIG.  311. — Vector  diagram  for  an  induction  furnace. 

295.  The  Constant-current  Transformer. — For  the  opera- 
tion of  arc  lamps  in  series,  the  constant-current  transformer 
shown  in  Fig.  312  is  used.  The  primary  coil  is  stationary  and 
receives  power  at  constant  potential,  while  the  secondary  coil, 
which  is  suspended  and  is  free  to  move  toward  or  away  from 
the  primary,  delivers  a  constant  current  to  the  lighting 
circuit. 

When  the  secondary  coil  is  close  to  the  primary,  the  react- 
ances of  the  transformer  are  small  and  the  secondary  voltage 
is  approximately  equal  to  the  primary  voltage  multiplied  by 
the  ratio  of  the  number  of  turns.  As  the  distance  between 
the  coils  is  increased,  the  leakage  -flux  and  the  reactances 
increase  and  the  secondary  voltage  drops,  even  although  the 
primary  voltage  remains  constant. 

The  primary  and  the  secondary  currents  are  opposite  in 
direction  and,  under  these  conditions,  the  primary  and  the 
secondary  coils  repel  one  another.  The  counterweight  on  the 
secondary  coil  is  so  adjusted  that,  when  the  desired  current  is 


268    PRINCIPLES  OF  ELECTRICAL  ENGINEERING  [CHAP,  xxxiv 


flowing  in  this  coil,  the  force  of  repulsion  keeps  the  secondary 
coil  suspended.  If  then  some  of  the  lamps  in  the  circuit  are 
cut  out,  the  current  tends  to  increase,  but  any  increase  in  the 
current  separates  the  coils  and  the  voltage  drops  due  to  the 
increased  reactance  and  so  is  unable  .to  maintain  the  current 
at  the  increased  value.  The  power  factor  of  such  a  transformer 


FIG.  312. — Constant-current  transformer. 

is  high  when  the  coils  are  close  together  but  decreases  as  the 
distance  between  the  coils  is  increased. 
296.  The  efficiency  of  a  transformer 

output  _  output 

input   ""  output  +  losses 

where  the  losses  are : 

Iron  losses;  the  hysteresis  and  eddy  current  losses  in  the  core. 
Copper  losses;  these  are  I^Ri  and  722#2  watts  respectively  for 


ART.  300] 


TRANSFORMER  CHARACTERISTICS 


269 


the  primary  and  secondary  windings.  There  is  no  power  loss  due 
to  the  primary  and  secondary  reactances,  see  page  209. 

297.  Hysteresis  Loss. — Since  the  flux  in  a  transformer  core 
is  alternating,  power  is  required  to  continually  reverse  the  mole- 
cular magnets  of  the  iron,  this  power  is  called  the  hysteresis  loss. 

298.  Eddy  Current  Loss. — If  the  transformer  core  in  Fig.  313 
is  made  of  a  solid  block  of  iron,  then  the  alternating  flux  <£  thread- 
ing this  core  causes  currents  to  flow  as  shown  at  Ay  in  the  same 
way  as  through  a  short-circuited  secondary  winding.     Power  is 
required   to    maintain   these    eddy    currents 

which  power  is  called  the  eddy  current  loss. 

To  keep  these  eddy  currents  small,  a  high 
resistance  is  placed  in  their  path.  This  is 
accomplished  by  laminating  the  core,  as  shown 
at  B,  the  laminations  being  separated  from  one 
another  by  varnish. 

299.  Iron  Losses. — The  hysteresis  and  the 
eddy  current  losses  taken  together  constitute 
what  is  called  the  iron  loss.     Since  the  flux 
per  pole  is  practically  constant  at  all  loads, 
see  page  262,  therefore  the  iron  loss  caused  by 
this  flux  is  also  constant  at  all  loads.     This 
loss  may  readily  be  determined  by  operating 

the  transformer  at  no-load  with  normal  voltage  and  frequenc}^, 
the  input  under  these  conditions,  measured  by  a  wattmeter,  is 
equal  to  the  iron  loss,  the  small  copper  loss  due  to  the  no-load 
current  being  neglected. 

300.  The  all-day  efficiency  is  defined  as  the  ratio  of  the  total 
energy  used  by  the  customer  to  the  total  energy  input  to  the  trans- 
former, during  twenty-four  hours.     This  efficiency  is  of  impor- 
tance in  the  case  of  lighting  transformers,  which  are  connected 
to  the  mains  for  twenty-four  hours  a  day  but  which  supply 
energy  for  about  five  hours  a  day,  under  such  conditions  the  all- 

,        ffl  . output  x  h 

"output  xh  +  iron  loss  X  24  +  copper  loss  x  h 
where  h  is  the  number  of  hours  per  day  during  which  energy  is 
taken  from  the  transformer. 

In  a  50-kv.a.  2200-  to  220-volt  transformer  the  iron  loss  is  300  watts,  the 
primary  resistance  is  0.5  ohms  and  the  secondary  resistance  is  0.005  ohms. 
Find 

a.  The  efficiency  when  the  load  is  50  kw.  and  the  power  factor  100  per 
cent. 


FIG.    313.— Trans- 
former core. 


270    PRINCIPLES  OF  ELECTRICAL  ENGINEERING    [CHAP,  xxxiv 

6.  The  efficiency  when  the  load  is  5  k\v.  and  the  power  factor  100  per 
cent. 

c.  The  efficiency  when  the  load  is  50  kv.a.  and  the  power  factor  80  per 
cent. 

d.  The  all-day  efficiency  in  the  latter  case  if  the  load  is  constant  and  con- 
nected for  5  hours  a  day  while  the  transformer  is  connected  to  the  line  for 
24  hours  a  day. 

a.  72,  the  secondary  current  =  50000/220  =  227  amp. 

/i,   the   primary   current  =  50000/2200  =  22.7  amp.    approximately 

Copper  loss  =  2272  X  0.005  +  22. 72  X  0.5  -  514  watts. 

Iron  loss  =  300  watts 

Total  loss  =814  watts 

Output  =  50,000  watts 

Input  =  50,814  watts 

Efficiency  =  98. 5  per  cent. 

b.  72  =  5000/220  =  22.7  amp.  and  /i  =  5000/2200  =  2.27  amp.  approx. 

Copper  loss  =  22.72  X  0.005  +  2.272  X  0.5  =  5.14  watts 

Iron  loss  =  300  watts 

Total  loss  =  305  watts 

Output  =  5000  watts 

Input  =  5305  watts 

Efficiency  =  94.4  per  cent. 

c.  7  2  =  50000/220  =  227  amp.  and /i  =  50000/2200  =  22.7  amp.  approx. 

Copper  loss  =  2272  X  0.005  -f  22. 72  X  0.5                   =  514  watts 

Iron  loss  =  300  watts 

Total  loss  =  814  watts 

Output  =  40,000  watts 

Input  =  40,814  watts 

Efficiency  =  98  per  cent. 

40000  X  5 
rf.  All-dav  efficiency  = 


40000  X  5  +  300  X  24  +  514  X  5 
=  95.5  per  cent. 

301.  Cooling  of  Transformers. — Transformers  become  heated 
up  due  to  the  losses.  This  heat  must  be  dissipated  and  the 
temperature  of  the  transformer  windings  kept  below  the  value 
at  which  the  insulation  begins  to  deteriorate.  Transformers  with 
an  output  of  less  than  1  kv.a.  can  dissipate  their  heat  by  direct 
radiation,  but  it  is  usual  to  place  all  transformers  up  to  500  kv.a. 
into  a  steel  tank,  which  is  then  filled  with  insulating  oil  above  the 
level  of  the  windings.  The  oil  improves  the  insulation,  and  con- 
vection currents  are  set  up  in  the  oil  by  means  of  which  the  heat 
is  carried  from  the  surface  of  the  transformer  to  the  larger  surface 
of  the  tank,  from  which  it  is  dissipated  to  the  surrounding  air. 
Such  a  transformer  is  said  to  be  self  cooled. 

The  losses  in  a  transformer  are  proportional  to  the  volume  of 


ART.  301] 


TRANSFORMER  CHARACTERISTICS 


271 


the  transformer,  while  the  radiating  surface  is  equal  to  the  super- 
ficial area,  so  that,  as  the  size  of  the  transformer  increases,  the 
losses  increase  more  rapidly  than  the  radiating  surface;  special 
arrangements  must  therefore  be  made  for  cooling  transformers  of 
large  output. 

For  outputs  from  50  to  500  kv.a.,  corrugated  tanks  such  as 
that  in  Fig.  314  are  used,  while  for  outputs  from  500  to  2000  kv.a., 


FIG.  314. — Corrugated  tank. 


FIG.  315. — Tank  with  external  cooling 
pipes. 


more  surface  must  be  provided  than  can  be  obtained  from  a  cor- 
rugated tank  and  the  construction  shown  in  Fig.  315  is -used;  for 
outputs  greater  than  2000  kv.a.,  other  methods  of  cooling  allow 
the  use  of  a  smaller  and  cheaper  transformer. 

The  water-cooled  type  is  shown  in  Fig.  316;  cold  water  is  cir- 
culated through  the  water  coil  and  takes  the  heat  from  the  hot 
upper  layers  of  the  oil.  About  2  1/2  Ib.  or  a  quarter  of  a  gallon 
of  water  is  required  per  minute  per  kilowatt  loss  in  the  transformer. 

The  air-blast  type  of  transformer  is  shown  in  Fig.  317.     The 


272    PRINCIPLES  OF  ELECTRICAL  ENGINEERING    [CHAP,  xxxiv 

transformer  is  well  supplied  with  ducts  so  that  the  air  can  reach 
the  points  at  which  the  heat  is  generated.  This  type  of  trans- 
former is  lighter  than  the  oil-filled  type  but  cannot  be  satis- 
factorily insulated  for  voltages  greater  than  30,000  volts.  One 
hundred  and  fifty  cubic  feet  of  air  is  required  per  minute  per 
kilowatt  loss. 


Air  Duct 


FIG.  316. — Diagram-  FIG.  317. — Diagrammatic  represen- 
matic  representation  of  a  tation  of  an  air-blast  transformer, 
water-cooled  transformer. 


FIG.  318. — Method  of  cooling  a  transformer  by  circulating  the  oil. 

The  circulating  oil  type  of  transformer  is  shown  in  Fig.  318 
and  may  be  used  with  advantage  where  only  hard  water  is  avail- 
able for  cooling  purposes.  When  such  water  is  used  in  water- 
cooled  transformers,  salts  are  liable  to  deposit  inside  the  cooling 
coils  and  throttle  the  supply  of  water.  With  the  circulating 
oil  method  of  cooling,  these  salts  will  deposit  on  the  outside  of  the 
cooling  coils. 


CHAPTER  XXXV 
TRANSFORMER  CONNECTIONS 

302.  Lighting  Transformers. — These  are  generally  built  to 
transform  from  2200  to  110  volts,  but  both  primary  and  secondary 
windings  are  divided  as  shown  diagrammatically  in  Fig.  319,  and 
these  windings  may  be  so  connected  that  a  standard  transformer 
can  operate  on  the  high-tension  side  at  either  2200  or  1100  volts 
and  on  the  low-tension  side  at  either  220  01  110  volts.  The  differ- 
ent connections  used  are  shown  in  Fig.  319. 


1100  Volts  to  110  Volts 


to  220  Volts 
3  Wire 


2200  Volts  to  110  Volts  to  220  Volts          to  220  Volts 

3  Wire 

FIG.  319. — Standard  connections  for  a  lighting  transformer. 

303.  Connections  to  a  Two -phase  Line. — In  Fig.  320,  diagram 
A  shows  the  method  of  transforming  from  high- voltage  two-phase 
to  low- voltage  two-phase,  for  the  operation  of  motors. 

Specify  the  transformers  for  a  two-phase  motor  which  delivers  50  h.p.  at 
440  volts  with  an  efficiency  of  90  per  cent,  and  a  power  factor  of  88  per  cent., 
the  line  voltage  being  2200. 

18  273 


274     PRINCIPLES  OF  ELECTRICAL  ENGINEERING     [CHAP,  xxxv 

motor  output  =  50  h.p. 

=  50  X  0.746  =  37.2  kw. 
motor  input     =  37.2/0.9  =  41.4  kw. 

=  41.4/0.88  =  47kv.a. 

therefore  two  transformers  are  required  each  of  23.5  kv.a.  output.  The 
secondary  current  =  23500/440  =  53  amp.,  the  primary  current,  neglect- 
ing transformer  losses  =  23500/2200  =  10.7  amp. 

Diagram  B  shows  the  method  of  connecting  a  low-voltage 
single-phase  load  to  a  two-phase  line;  the  load -should  be  divided 
as  equally  as  possible  between  the  two  phases. 

Diagram  C  shows  the  Scott  connection  used  to  transform 
from  two-phase  to  three  phase.  The  two  transformers  x  and  y 
are  wound  for  the  same  primary  voltage,  namely  that  of  the 
two-phase  line,  but  the  secondary  voltage  of  y  is  only  V3/2  or 


Two  rhuse  Generator 


A  -Two  "Phase 
Motor 


B- Single  Phase 
Load 


D  -  Voltages  in  a 
Scott  Connection 


FIG.  320. — Connection  of  transformers  to  a  two-phase  line. 

0.86  times  that  of  x.     One  end  of  the  secondary  of  y  is  connected. 

to  the  middle  point  of  x. 

If  the  secondary  voltage  of  x  is  E  volts  then  that  of  y  is  V3£"/2 

volts  and,  in  diagram  D,  Fig.  320, 

the  difference  of  potential  between  a  and  b  =  E  volts 

the  difference  of  potential  between  a  and  c  =  V(ao)2  +  (oc)2 

=  E  volts 
the  difference  of  potential  between  b  and  c  =  E  volts 

and  the  phase  relations  between  these  voltages  is  the  same  as  in 

a  delta-connected  bank  of  transformers. 

A  load  which  is  balanced  on  the  three-phase  side  is  also  balanced  on  the  two- 
phase  side.     Fig.  321  shoSvs  the  currents  and  voltages  in  a  Scott  connected 


ART.  303] 


TRANSFORMER  CONNECTIONS 


275 


group  which  is  used  to  transform  from  two-phase  at  E  volts  to  three-phase 
at  E  volts. 

Since  the  three-phase  load  is  balanced,  therefore  7i,72  and  73  are  all  equal, 
and  the  kv.a.  output  on  the  three-phase  side  =  1.73EI. 

Now  the  magnetizing  effect  of  the  primary  of  a  transformer  is  always 
equal  to  the  magnetizing  effect  of  the  secondary,  so  that  in  transformer  y 


and 


therefore 


n\y  X  Iy  =  nz,j  X  I 
niy  E 


Iv  =  I  X  A/3/2 


In  transformer  x,  the  current  in  oa  =  1  and  that  in  ob  also  =  7,  but  these 
currents  are  out  of  phase  by  120  degrees  and  are  in  opposite  directions  there- 


0000 

8  9 


la  -I 


r 

L  '2 


B 


FIG.  321. — Voltage  and  current  relations  in  a  Scott-connected  bank  of 

transformers. 


fore  the  resultant  magnetizing  effect  is  not  equal  to  n2xl  but  to  n2xl  X  \/3/2 
see  diagram  B 


nlx 
- 

7*2* 


E 


now  -  =  p  =  1 

7*2*  & 

and  nixlx  =  nzxl  X  \/3/2 

therefore       Ix  =  I  X  V3/2  =  Iy 

and  the  total  two-phase  kv.a.  =  2^7^  =  v/3£f7,  the  same  as  on  the  three- 
phase  side. 

If  a  50-h.p.,  440'volt,  three-phase  motor  is  supplied  from  a  2200  volt, 
two-phase  line,  find  the  current  7,  Fig.  321,  and  also  the  currents  Ix  and  Iv, 
if  the  efficiency  of  the  motor  is  90  per  cent,  and  the  power  factor  is  88  per 
cent. 


276     PRINCIPLES  OF  ELECTRICAL  ENGINEERING     [CHAP,  xxxv 


motor  output  =  50  h. p. 

=  50  X  0.746  =  37.2  kw. 
motor  input      =  37.2/0.9  =  41.4  kw. 

=  41.4/0.88  =  47  kv.a. 

1.73  XEXI, 

kv.a.,  Fig.  321 


1000 
1.73  X440  X7 


kv.a. 


and 


1000 
/  =  47,000/1.73  X  440  =  62  amp. 


neglecting    transformer    losses,    2   X  2200   X  /*    =47  kv.a., 
and    Ix  =  47,000/2  X  2200  =  10.7  amp. 

304.  Connections  to  a  Three-phase  Line. — In  Fig.  332: 
Diagram  A  shows  three  transformers  connected  Y  on  both 
primary  and  secondary  sides.  If  EI  is  the  voltage  between  the 


To  Generator 


Three  Phase  Motor 


¥ 

>y 


& 


r 

A 


A 
A 


FIG.  322. — Connection  of  transformers  to  a  three-phase  line. 

primary  lines  and  E2  is  that  between  the  secondary  lines  then 

The  primary  voltage  of  each  transformer  =  JSi/V3,  see  page  236. 

The  secondary  voltage  of  each  transformer  =  #2/V3. 

.The  transformation  ratio  of  each  transformer  =  E\/E*. 

Diagram  B  shows  the  transformers  connected  delta  on  both 
primary  and  secondary  sides: 

The  primary  voltage  of  each  transformer  =  EI,  see  page  237. 

The  secondary  voltage  of  each  transformer  =  E%. 

The  transformation  ratio  of  each  transformer  =  Ei/E*. 

Diagram  C  shows  the  transformers  connected  Y  on  the  prim- 
ary side  and  delta  on  the  secondary  side : 

The  primary  voltage  of  each  transformer  =  EVV3. 

The  secondary  voltage  of  each  transformer  =  E2- 


ART.  30 4] 


TRAN8FOR  MER  CONNECTIONS 


277 


The  transformation  ratio  of  each  transformer  = 
In  a  delta  connection,  the  voltage  of  any  one  phase  at  any 
instant  is  equal  and  opposite  to  the  sum  of  the  voltages  in  the 
other  two  phases,  see  page  233,  so  that  if,  in  Fig.  323,  one  trans- 
former A  of  a  bank  of  delta-connected  transformers  is  discon- 
nected, the  difference  of  potential  between  a  and  6  is  unchanged, 
being  maintained  by  the  transformers  B  and  C  in  series,  so  that 
three-phase  power  can  still  be  obtained  from  the  lines  a,  b  and  c. 
This  connection  is  called  the  V  or  open  delta  connection  and  is 
shown  in  diagram  D,  Fig.  322. 


Primary  Secondary 

Delta  connection. 


Primary 

Open-delta  connection. 

FIG.  323. — Comparison    between    the    closed-delta    and    the    open-delta 

connections. 

In  the  open  delta  connection  in  Fig.  323. 

The  current  in  each  transformer  =  Ii. 
The  voltage  of  each  transformer  =  Et 


The  rating  of  each  transformer  = 


EJi 

1000 


kv.a. 


1  7^?  TT  T 

But  the  total  load  on  the  three-phase  line  =  ~  IQQQ     kv.a., 

page  237,  so  that  the  rating  of  each  transformer  is  greater  than 
half  the  total  load  by  the  ratio  2/1.73  or  by  15  per  cent. 

If  30  kv.a.  has  to  be  transmitted,  then  three  delta-connected  transformers 
may  be  used  each  with  a  capacity  of  10  kv.a. 


278     PRINCIPLES  OF  ELECTRICAL  ENGINEERING     [CHAP,  xxxv 

If  one  of  these  transformers  is  burnt  out,  then  the  power  that  can  be  trans- 
mitted by  the  remaining  two  transformers  is  not  20  kv.a.,  but 

=  20  X  V3/2  =  17.3  kv.a. 

336.  Advantages  and  Disadvantages  of  the  Y  and  Delta  Con- 
nection.— For  a  given  voltage  of  110,000  volts  between  lines, 
each  transformer  in  a  Y- connected  bank  has  to  withstand 
110,000/V3  or  64,000  volts  and  is  therefore  less  liable  to  break 
down  than  the  transformers  in  a  delta-connected  bank  which 
have  to  withstand  the  full  110,000  volts. 

When  one  transformer  in  a  Y-connected  bank  breaks  down,  the 
system  can  no  longer  operate  three-phase,  whereas,  if  the  trans- 
formers were  connected  delta,  the  faulty  transformer  could  be 


26  Amp, 


16 
Amp.' 


15 
Amp.' 


Amp. 


A-  Connection  Diagram 

FIG.  324. 


20 
Amp. 


B-  Vector  'Diagram 


disconnected  and  about  58  per  cent,  of  the  load  could  still  be 
carried  by  the  two  remaining  transformers  connected  open  delta. 
Delta-connected, transformers  are  used  for  practically  all  low 
voltage  distribution.  The  Y  to  delta  connection  is  often  used 
for  higV voltage  work,  the  transformers  being  Y  connected  on  the 
high-voltage  side. 


The  load  on  a  2200-volt  three-phase  line  consists  of  1800,  1/2  amp.,  110 
volt  lamps  on  three  circuits,  and  200  h.p.  of  three-phase  motors  with  an 
average  power  factor  of  80  per  cent,  and  an  average  efficiency  of  88  per  cent, 
all  on  one  circuit.  Draw  the  diagram  of  connections,  specify  the  trans- 
formers, and  find  the  current  ^n  the  mains,  and  also  the  resultant  power 
factor.  The  connection  diagram  is  shown  in  Fig.  324. 


ART.  307]  TRANSFORMER  CONNECTIONS  279 

The  kv.a.  output  of  each  motor  transformer 
=  l/3(motor  kv.a.) 
=  1/3(200  X  0.746  X  Q  ^  X  ^g) 

=  70  kv.a. 

3  X  70  X  1000 
the  line  current  for  the  motors   =  ~T^7o~croonn"    =  ^5  amp. 

l./o    /\  ZZUU 


the  kv.a.  output  of  each  lighting  transformer 

=  600  X  0.5  X  110 
1000 

=  33  kv.a. 

33  000 
the  primary  current  in  each  transformer  =    nknT    =   15  amp.,  but  these 


transformers  form  a  delta-  connected  load  on  the  line,  see  Fig.  269,  page  237, 
therefore  the  current  in  each  line  =  1.73  X  15  =  26  amp. 
the  resultant  current  in  the  line  is  the  resultant  of  26  amp.  at  100  per  cent. 
power  factor  and  of  55  amp.  at  80  per  cent,  power  factor  and  is  equal  to 


/  =    Va62  +  6c2,  diagram  B 

=  V(0.8  X  55  +  26)2  +  (0.6  X  55)2 
=  77.5  amp 

ab       0.8  X  55  +  26 
the  power  factor  =  —  =  - 


=  90.5  per  cent. 

306.  Types  of  Transformer.  —  The  transformer  shown  in  Fig. 
325  is  said  to  be  of  the  core  type.     If  the  coil  on  limb  B  is  placed 
on  A,  and  the  iron  of  B  is  split  and  bent  over  as  shown  in  Fig.  326, 
the  resulting  transformer  is  said  to  be  of  the  shell  type. 

For  three-phase  transmission  there  is  a  considerable  saving  in 
cost  and  floor  space  if,  instead  of  a  bank  of  three  transformers 
each  in  its  own  tank,  a  three-phase  transformer  is  used  in  which 
the  windings  of  the  three  phases  are  all  placed  on  the  same  core 
as  shown  in  Figs.  327  and  328.  The  principal  objection  to  the 
three-phase  transformer  is  that  a  breakdown  in  the  winding  of 
one  phase  puts  the  whole  transformer  out  of  commission,  so  that 
such  transformers  are  used  only  in  large  central  stations  where 
there  is  ample  reserve  capacity. 

307.  The  Autotransformer.  —  If  a  reactance  coil  is  placed  across 
an  alternating-current  line  as  shown  in  Fig.  329,  the  voltage  E2 
obtained  by  tapping  the  coil  as  shown  may  have  any  value  less 

ET  r 

than  EI,  and,  as  in  the  transformer,  -~l-  =       =  }.    Since  the 


280.    PRINCIPLES  OF  ELECTRICAL  ENGINEERING     [CHAP,  xxxv 

primary  and  secondary  currents  oppose  one  another,  the  current 
in  the  section  ab  is  the  difference  between  the  currents  /i  and  72. 


FIG.  325.— Core  type  of  trans- 
former. 


FIG.  326. — Shell  type  of  trans- 
former. 


Core 


Secondary 


FIG.  327. — Three-phase  core 
type  of  transformer. 


FIG.  328.— Three-phase  shell 
type  of  transformer. 


n<i  Turns 
-1* 


FIG.  329. — Autotransformer. 


When  the  transformation  ratio  is  comparatively  small,  the 
autotransformer,  as  this  reactance  coil  is  called,  is  cheaper  than 


AKT.  308] 


TRA NSFOHMER  CONNECTIONS 


281 


the  equivalent  transformer  with  separate  windings.  It  is  much 
used  for  reducing  the  voltage  applied  to  alternating-current 
motors  during  the  starting  period,  see  page  301. 

308,  Boosting  Transformers  and  Feeder  Regulators. — The 
voltage  of  a  line  may  be  raised  by  a  small  amount  if  a  standard 
transformer  is  connected  as  shown  in  Fig.  330.  The  voltage  of 
the  line  is  boosted  by  the  amount  of  the  secondary  voltage  E2. 


FIG.  330. — Boosting  transformer. 

When  the  secondary  side  of  the  transformer  is  tapped  so  that 
the  boosting  effect  can  be  adjusted,  the  resulting  piece  of  appara- 
tus is  the  Stillwell  feeder  regulator.  It  is  used  to  raise  the  feeder 
voltage  above  that  of  the  power  house,  so  as  to  compensate  for 
the  voltage  drop  in  the  feeder  and  maintain  the  voltage  at  the 
load. 

Another  type  of  feeder  regulator  is  shown  in  Fig.  331.     The 


Induction  regulator. 


secondary  coil  in  this  case  is  movable  relative  to  the  primary. 
When  the  coils  are  in  the  relative  position  shown  in  diagram  A, 
the  voltage  E2  has  its  maximum  value;  when  the  secondary  coil 
is  moved  through  90  degrees  relative  to  the  primary,  as  shown  in 
diagram  B,  then  the  flux  <f>  which  threads  the  secondary  coil  is 
zero,  and  no  voltage  is  induced  in  that  coil.  Such  regulators  may 
be  made  automatic  if  the  core  is  turned  by  means  of  a  small  motor 


282    PRINCIPLES  OF  ELECTRICAL  ENGINEERING      [CHAP,  xxxv 

which  is  controlled  by  a  solenoid  connected  across  the  line  to  be 
regulated.  When  the  line  voltage  increases,  the  plunger  of  the 
solenoid  is  raised  and  closes  the  motor  circuit,  and  the  motor 
then  turns  the  regulator  in  such  a  direction  as  to  lower  the  volt- 
age. When  the  line  voltage  decreases,  ttfc  plunger  of  the  solenoid 
drops  and  reverses  the  motor,  so  that  it  now  turns  the  regulator 
in  the  opposite  direction. 


CHAPTER  XXXVI 
POLYPHASE  INDUCTION  MOTORS 

309.  The  induction  motor  is  used  for  practically  all  the  power 
work  when  only  alternating  current  is  available.     The  essential 


FIG.  332. — Squirrel-cage  induction  motor. 

parts  of  such  a  machine  are  shown  in  Fig.  332.     The  stator  or 
stationary  part  is  exactly  the  same  as  that  of  an  alternator,  the 


FIG.  333. — Squirrel-cage  rotor. 

rotor,  however,  is  entirely  different  and  the  type  most  generally 
used,  called  the  squirrel-cage  type,  consists  of  a  cylindrical  core 

283 


284    PRINCIPLES  OF  ELECTRICAL  ENGINEERING    [CHAP,  xxxvi 

which  carries  a  large  number  of  copper  bars  on  its  periphery 
which  bars  are  all  joined  together  at  the  ends  by  copper  or  brass- 
end  connectors  as  shown  in  Fig.  333.  The  action  of  this  machine 
will  be  explained  in  detail. 

310.  The  Revolving  Field. — Diagram  P,  Fig.  334,  shows  the 
essential  parts  of  a  two-pole,  two-phase  induction  motor.  The 
stator  carries  two  windings  M  and  N  which  are  spaced  90  elec- 
trical degrees  apart  and,  in  the  actual  machine,  are  bent  back  so 
that  the  rotor  may  readily  be  inserted.  These  windings  are 


Ph-l    ph-2 


FIG.  334. — Revolving  field  of  a  two-pole,  two-phase  induction  motor. 

connected  by  wires  to  two-phase  mains  and  the  currents  which 
flow  at  any  instant  in  the  coils  M  and  N  are  given  by  the  curves 
in  diagram  Q;  at  instant  A  for  example  the  current  in  phase  1  = 
+  Im  while  that  in  phase  2  is  zero. 

The  windings  of  each  phase  are  marked  S  and  F  at  the  terminals 
and  these  letters  stand  for  start  and  finish  respectively;  a  + 
current  is  one  that  goes  in  at  S  and  a  —  current  one  that  goes  in 
atF. 

The  resultant  magnetic  field  produced  by  the  windings  M  and 
N  at  instants  A,  B,  C  and  D  is  shown  in  diagram  R  from  which  it 
may  be  seen  that,  although  the  windings  are  stationary,  a  revolv- 
ing field  is  produced  which  is  of  constant  strength  and  which 


ART.  311] 


POLYPHASE  INDUCTION  MOTORS 


285 


goes  through  one  revolution  while  the  current  in  one  phase  passes 
through  one  cycle. 

311.  The  Revolving  Field  of  a  Three-phase  Motor. — Diagram 
P,  Fig.  335,  shows  the  winding  for  a  two-pole  three-phase  motor; 
M,  N  and  Q,  the  windings  of  the  three  phases,  are  spaced  120 
electrical  degrees  apart.  These  windings  are  connected  to  the 
three-phase  mains  and  may  be  connected  either  Y  or  delta,  see 
page  237,  in  either  case  the  currents  which  flow  at  any  instant  in 


Ph.l  Ph.2  Ph.3 


13*  -  Im 
FIG.  335. — Revolving  field  of  a  two-pole,  three-phase  induction  motor. 

the  coils  M,  N  and  Q  are  given  by  the  curves  in  diagram  R;  at 
instant  A  for  example  the  current  in  phase  1  =  -\-Im  that  in  phase 

2  =  -     2*  and  that  in  phase  3  is  also  =  -•-£' 

The  resultant  magnetic  field  produced  by  the  windings  at 
instants  A,  B,  C  and  D  is  shown  in  diagram  S  from  which  it  may 
be  seen  that,  just  as  in  the  case  of  the  two-phase  machine,  a 
revolving  field  is  produced  which  is  of  constant  strength  and 
which  goes  through  one  revolution  while  the  current  in  one  phase 
passes  through  one  cycle. 


286    PRINCIPLES  OF  ELECTRICAL  ENGINEERING    [CHAP,  xxxvi 

Si 


FIG.  336. — Revolving  field  of  an  eight-pole,  two-phase  induction  motor. 


ART.  313] 


POLYPHASE  INDUCTION  MOTORS 


287 


312.  Multipolar  Machines. — Fig.  336  shows  part  of  the  wind- 
ing of  an  eight-pole,  two-phase  induction  motor  and  also  the  re- 
sultant magnetic  field  at  the  instants  A  and  B,  Fig.  334.  The 
field  moves  through  the  angle  6,  or  through  half  the  distance 

between  two  adjacent  poles  or  0     of  a  revolution,  in  the  time 

zp 

interval  between  the  instants  A  and  B  or  in  r,  seconds,  therefore 
the  speed  of  the  revolving  field 

=  o~  X  4/  rev.  per  sec. 


12Qf 
P 


rev  per.  mm. 


This  is  called  the  synchronous  speed,  and  is  the  speed  at  which 
an  alternator  with  the  same  number  of  poles  must  be  run  in  order 
to  give  the  same  frequency  as  that  applied  to  the  motor,  the  table 
on  page  195  therefore  applies  to  induction  motors  as  well  as  to 
alternators. 

313.  The  Starting  Torque.— Let  the  revolving  field  of  a  two- 
pole  machine,  produced  by  either  a  two-  or  a  three-phase  stator,' 


FIG.  337.— Direction 
of  the  e.m.f.  in  the 
rotor  bars. 


FIG.  338. — Direction 
of  the  currents  in  the 
bars  of  a  low  resistance 
rotor. 


FIG.  339.— Direction 
of  the  currents  in  the 
bars  of  a  high  resistance 
rotor. 


be  represented  by  a  revolving  north  and  south  pole  as  shown  in 
Fig.  337.  The  field  is  moving  in  the  direction  of  the  arrow  and 
therefore  cuts  the  stationary  rotor  bars  and  generates  in  them 
e.m.fs.  which  are  shown  by  crosses  and  dots.  The  e.m.f.  will  be 
a  maximum  in  the  conductor  which  is  in  the  strongest  field  and  the 
direction  of  the  e.m.f.  in  each  conductor  may  be  found  by  the  right 
hand  rule,  see  page  192.  Since  the  rotor  winding  forms  a  closed 


288    PRINCIPLES  OF  ELECTRICAL  ENGINEERING    [CHAP,  xxxvi 

circuit,  the  generated  e.m.fs.  will  cause  currents  to  flow  in  the 
rotor  bars. 

The  frequency  of  the  e.m.fs.  generated  in  the  stationary  rotor 

bars  by  the  revolving  field  =  —        <wr'         >  see  page  195)  which 


is  the  same  as  the  frequency  of  the  e.m.f.  applied  to  the  stator, 

f  X  120 
since  the  syn.  speed  =  -  —  ,  see  page  287,   so  that  the  fre- 

quency of  the  rotor  currents  is  high,  and  the  reactance  of 
the  rotor,  which  is  proportional  to  the  rotor  frequency,  is 
large  compared  with  the  rotor  resistance,  the  rotor  current  in 
each  bar  therefore  lags  considerably  behind  the  e.m.f.  in  that  bar. 
In  conductor  a  for  example,  the  e.m.f.  has  just  reached  its  maxi- 
mum value,  but  the  current  in  that  conductor  lags  the  e.m.f.  by 
an  angle  6  which  is  almost  90  degrees  and  so  does  not  become  a 
maximum  until  the  poles  have  moved  into  the  position  shown  in 
Fig.  338,  which  is  almost  90  degrees  from  the  position  shown  in 
Fig.  337. 

Since  the  conductors  in  Fig.  338  are  carrying  current  and  are  in 
a  magnetic  field,  they  are  acted  on  by  forces,  the  direction  of  which 
may  be  determined  by  the  left-hand  rule,  from  which  it  is  found 
that  while  the  force  in  the  belts  be  and  de  tends  to  make  the  rotor 
follow  the  revolving  field,  that  on  the  conductors  in  the  belts  cd 
and  be  acts  in  the  opposite  direction.  The  former  force  is  the 
larger,  so  that  the  rotor  tends  to  follow  the  revolving  field. 

The  current  in  the  rotor  conductors  at  standstill 

rotor  voltage  at  standstill 
rotor  impedence  at  standstill 

and  the  rotor  impedence  is  large  enough  to  limit  the  current  to 
about  five  times  the  full-load  value.  On  account  of  the  large 
torque  opposing  the  starting  of  the  rotor,  this  large  starting  cur- 
rent produces  an  effective  starting  torque  which  seldom  exceeds 
one  and  a  half  times  the  full-load  torque  of  the  motor. 

314.  The  Wound  Rotor  Motor.  —  The  starting  torque  for  a 
given  current  can  be  increased  if  that  current  is  brought  more  in 
phase  with  the  e.m.f.  This  may  be  seen  from  Fig.  339  in  which 
the  current  lags  by  such  a  small  angle  6  that  there  is  practi- 
cally no  opposing  torque. 

The  angle  of  lag  in  a  circuit  may  be  decreased  by  increasing  the 
resistance  of  the  circuit  without  changing  the  value  of  the  react- 


ART.  314] 


POLYPHASE  INDUCTION  MOTORS 


289 


ance.  In  Fig.  340  for  example,  when  the  resistance  R  is  in- 
creased, the  vector  diagram  changes  from  A  to  B]  the  current  1 
is  decreased  and  so  also  is  the  angle  6.  If  then  sufficient  resist- 


"   E 

JL 


A  B 

FIG.  340. — Effect  of  resistance  on  the  magnitude  and  on  the  phase  angle  of 
current  in  a  series  circuit. 

ance  is  put  into  the  rotor  bars  to  reduce  the  starting  current  to 
the  full-load  value,  the  angle  of  lag  of  the  rotor  currents  will  be  so 
small  that  the  torque  is  all  effective,  and  full-load  torque  is 


.Eotor  Bar 


Rotor 


rFA/W= 


Slip  King 


FIG.  341.— Squirrel-cage  rotor  with  bars  that  have  an  adjustable  resistance. 


FIG.  342.— Wound  rotor. 

developed  with  full-load  current.  By  using  a  lower  resistance, 
twice  full-load  torque  may  be  obtained  with  about  twice  full- 
load  current. 

19 


290    PRINCIPLES  OF  ELECTRICAL  ENGINEERING    [CHAP,  xxxvi 

When  a  motor  is  running  under  load,  a  large  rotor  resistance  is 
undesirable  because  it  causes  large  copper  loss,  low  efficiency  and 
excessive  heating,  so  that  some  method  must  be  devised  whereby 
resistance  can  be  inserted  in  the  rotor  conductors  during  the  start- 
ing period  and  cut  out  during  the  running  period. 

This  result  could  be  attained  by  leaving  the  rotor  bars  open  at 
one  end  and  connecting  that  end  to  a  slip  ring  on  the  shaft, 
as  shown  in  Fig.  341,  between  which  ring  and  the  stationary 
end  connector  C  an  adjustable  resistance  R  could  be  inserted. 
This  construction  would  necessitate  the  use  of  as  many  slip 
rings  as  there  are  rotor  bars  so  that  it  is  modified  in  practice  to 
reduce  the  number  of  slip  rings. 

The  bars  are  connected  so  as  to  form  a  winding  of  three  groups, 
as  shown  diagrammatically  in  Fig.  342.  One  end  of  each  group 
is  connected  to  the  end  connector  Ci,  while  the  free  ends  are 
connected  to  slip  rings  and  then  through  adjustable  resistances 
Ri  to  the  other  end  connector  (7,  which  is  now  merely  a  short 
piece  of  wire  connecting  the  resistances  together.  The  resistances 
Ri  are  gradually  cut  out  as  the  motor  comes  up  to  speed,  and  are 
finally  short- circuited.  This  type  of  motor  is  called  the  wound 
rotor  type  of  induction  motor  and  is  to  be  preferred  to  the  squirrel 
cage  type  for  heavy  starting  duty. 

315.  Running  Conditions. — It  was  shown  on  page  288  that 
the  resultant  starting  torque  is  in  such  a  direction  as  to  make 
the  rotor  follow  up  the  revolving  field.  When  the  motor  is  not 
carrying  any  load,  the  rotor  will  revolve  at  practically  the  speed 
of  the  revolving  field,  that  is  at  synchronous  speed;  it  can  never 
run  at  a  speed  greater  than  that  of  the  revolving  field.  If  the 
motor  is  then  loaded,  it  will  slow  down  and  slip  through  the 
revolving  field,  the  rotor  bars  will  cut  lines  of  force,  the  e.  m.  fs. 
generated  in  these  bars  will  cause  currents  to  flow  in  them  and  a 
torque  will  be  produced.  The  rotor  will  slow  down  until  the  point 
is  reached  at  which  the  torque  developed  by  the  rotor  is  sufficient 
to  overcome  the  retarding  torque  of  the  load. 

syn.  r.p.m.  —  r.p.m.  of  rotor  . 

The  ratio—  -  is  called  the  per  cent, 

syn.  r.p.m. 

slip,  and  is  represented  by  the  symbol  s,  its  value  at  full-load  is 
generally  about  4  per  cent.,  that  is,  when  the  revolving  field 
make  one  revolution,  the  rotor  makes  0.96  of  a  revolution  and 
the  field  moves  relative  to  the  rotor  bars  through  0.04  of  a 
revolution. 


ART.  316] 


POLYPHASE  INDUCTION  MOTORS 


291 


When  the  rotor  is  at  standstill,  the  rotor  frequency  is  /  cycles 
per  second,  see  page  288,  where  /  is  the  frequency  of  the  e.m.f. 
applied  to  the  stator,  but  at  full-load  the  rotor  frequency  is  only 
sf  cycles  per  second  because  then  the  velocity  of  the  field  relative 
to  the  rotor  is  only  s  per  cent,  of  the  relative  velocity  at  standstill. 

As  the  load  on  the  motor  increases,  the  motor  slows  down  and 
the  rotor  slips  more  rapidly  through  the  revolving  field  and  causes 
the  rotor  current  to  increase,  but  the  frequency  of  the  rotor  cur- 
rent increases  as  well  as  its  numerical  value  and  this  causes  the 
rotor  reactance  to  increase  and  the  current  to  lag.  Now  the 
torque  developed  by  the  rotor  tends  to  increase  due  to  the  increase 
in  the  rotor  current  and  to  decrease  due  to  the  increase  in  the 


2CO 


600     800  1000  1200  1400 
E.P.M. 


Starting  current    =  160  amp.  =  5  times  full  load  current 
Starting  torque     =  1.9  times  full  load  torque 
Maximum  torque  =3.1  times  full  load  torque 

FIG.  343. — The  relation  between  torque,  current  and  speed  in  a  25  h.p., 
440  volt,  3-phase,  60  cycle  induction  motor. 

current  lag,  see  page  288.  Up  to  a  certain  point,  called  the  break- 
down point  or  point  of  maximum  torque,  the  effe'ct  of  the  current 
is  greater  than  that  of  the  current  lag,  beyond  that  point  the 
effect  of  the  current  lag  is  the  greater,  so  that,  after  the  break- 
down point  is  passed,  the  torque  actfu£lly  decreases  even  although 
the  current  is  increasing.  The  relation  between  speed,  torque  and 
current  is  shown  in  Fig.  343  for  a  25-h.p.,  440  volt,  three-phase, 
60-cycle,  1200  syn.  r.p.m  induction  motor. 

316.  Vector  Diagrams  for  the  Induction  Motor. — a.  No-load 
conditions.  Alternating  e.m.fs.  EI,  applied  to  the  stator,  cause 
alternating  currents  70  to  flow  in  the  stator  windings  and  pro- 
duce the  revolving  field  </>,  but  no  work  is  done  if  the  no-load 
losses  are  neglected,  so  that  the  current  flowing  in  each  phase 
must  lag  the  voltage  applied  to  that  phase  by  90  degrees,  as 


292    PRINCIPLES  OF  ELECTRICAL  ENGINEERING    [CHAP,  xxxvi 


shown  in  Fig.  344.  This  no  load  current  is  kept  as  small  as  pos- 
sible by  the  use  of  a  small  air  gap  clearance  between  the  stator 
and  rotor;  a  50-h.p.,  900-r.p.m.  induction  motor  will  have  a  rotor 
diameter  of  20  in.  (51  cm.)  and  an  air  gap  clearance  of  0.03 
in.  (0.075  cm.)  and,  in  spite  of  this  small  air  gap,  the  magnetizing 
current  70  will  not  be  less  than  30  per  cent,  of  full-load  current. 
The  revolving  field,  in  addition  to  cutting  the  rotor  conductors, 
cuts  also  those  of  the  stator  and  generates  an  e.m.f .  Eib,  called  the 


FIG.  344.— No-load      FIG.  345.— Ideal.     FIG.  346.— Actual, 
vector  diagram.  Full-load  vector  diagrams. 


10G 


80 


A 


Full  Load 

FIG.  347. — The  power  factor  of  an  induction  motor. 

back  e.m.f.,  in  each  phase  of  the  stator  winding.  Eib  is  less 
than  EI  by  the  e.m.f.  required  to  send  the  magnetizing  current 
/o  through  the  impedence  of  the  stator  winding,  which  e.m.f.  is 
comparatively  small.  The  revolving  field  is  therefore  propor- 
tional to  the  voltage  EI. 

b.  Full-load  conditions.     When  the  motor  is  loaded  it  slows 
down  and  current  passes  through  the  rotor  windings  and  develops 


ART.  317]  POLYPHASE  INDUCTION  MOTORS  293 

the  driving  torque  of  the  machine.  This  current,  like  that  in  the 
secondary  of  a  transformer,  tends  to  demagnetize  the  machine  or 
to  oppose  the  flux  which  produces  it,  but  a  reduction  in  the 
flux  <j>  causes  the  back  e.m.f.  Elb  to  decrease  and  allow  a  larger 
current  to  flow  in  the  primary  or  stator  winding,  so  that  the 
stator  current  always  adjusts  itself  to  suit  the  requirements  of 
the  secondary  or  rotor. 

The  reduction  in  the  value  of  <£  is  comparatively  small,  so  that 
at  full-load,  a  magnetizing  component  of  current  70  is  still  required 
to  produce  the  revolving  field  while  a  power  component  /„,  in 
phase  with  the  applied  voltage  is  required  for  the  load,  as  shown  in 
Fig.  345.  The  power  factor  of  an  induction  motor,  namely,  the 
cos  a,  is  therefore  practically  zero  at  no-load  and  gradually 
increases  as  the  load  increases,  as  shown  in  curve  Ay  Fig.  347. 

Due  to  the  reactance  of  the  stator  winding,  and  to  that  of  the 
rotor  winding  which,  as  pointed  out  on  page  291,  increases  with 
the  load,  the  current  lags  more  than  shown  in  Fig.  345  and  the 
power  factor  in  an  actual  machine  changes  with  the  load  as  shown 
in  curve  B. 

317.  Adjustable  Speed  Operation. — The  squirrel  cage.motor  is 
essentially  a  constant  speed  machine.  For  adjustable  speed 
operation  the  wound  rotor  motor  must  be  used.  If  such  a  motor 
is  operating  with  a  load  having  a  constant  torque,  and  a  resistance 
is  inserted  in  the  rotor  circuit,  the  rotor  current  will  decrease  and 
the  motor  will  not  be  able  to  develop  the  necessary  torque  and  so 
will  slow  down.  As  the  speed  drops  however  the  rotor  slips  more 
rapidly  through  the  revolving  field,  so  that  a  greater  e.m.f.  is 
induced  in  the  rotor  conductors  and  the  current  increases,  and, 
at  some  lower  speed,  is  again  sufficient  for  the  load. 

The  speed  regulation  however  is  very  poor  because,  if  the  load 
were  decreased,  less  current  would  be  required,  and  the  motor 
would  automatically  speed  up  so  as  to  reduce  the  slip  and 
thereby  reduce  the  rotor  voltage  and  current.  At  no-load, 
the  rotor  current  being  then  very  small,  the  slip  would  be 
practically  zero  and  the  motor  would  run  at  practically  syn- 
chronous speed.  This  method  of  speed  control  is  therefore 
similar  to  that  of  a  direct- current  shunt  motor  with  resist- 
ance in  the  armature  circuit  and  has  the  objection  that  the  speed 
regulation  is  poor  and  the  efficiency  is  low. 

Where  good  speed  regulation  is  desired,  the  stator  may  be  sup- 
plied with  two  separate  windings,  one  of  which  is  wound  so  as  to 


294   PRINCIPLES  OF  ELECTRICAL  ENGINEERING     [CHAP,  xxxvi 

produce  a  revolving  field  with  p  poles  and  the  other  a  revolving 
field  with  p1  poles  and  then,  by  using  one  or  other  of  these  wind- 

,       120/         120/       r^    . 

ings,  the  synchronous  speed  may  be  or  -   — .      It   is    sel- 

dom possible  on  account  of  expense  to  put  more  than  two  such 
windings  on  one  stator. 

318.  Induction  Generator. — Suppose  that  an  electric  car  driven 
by  an  induction  motor  is  operating  on  a  road  where  there  is  a 
long  pull  up  followed  by  a  long  coast  down.  When  on  the  up- 
grade, the  motor  runs  at  about  4  per  cent,  less  than  synchronous 
speed,  at  which  speed  the  rotor  slips  fast  enough  through  the 
revolving  field  to  cause  full-load  current  to  flow. 

Starting  on  the  down  grade,  the  car  begins  to  drive  the  motor 
and  the  speed  of  the  motor  first  becomes  equal  to  the  synchronous 
speed,  when  the  rotor  current  is  zero,  and  then  runs  at  a  speed 
which  is  greater  than  synchronous  so  that  the  rotor  is  again  slip- 


FIG.  348. — The  rotor  of  a  self-starting  synchronous  motor. 

ping  through  the  revolving  field.  But  since  it  is  now  running 
faster  than  the  field,  the  direction  of  motion  of  the  conductors 
relative  to  the  field  has  been  reversed,  and  the  torque  which  was 
a  driving  torque  now  becomes  a  retarding  torque  so  that  the 
machine  is  now  acting  as  a  generator  and  delivering  power  to  the 
mains.  When  the  speed  is  about  4  per  cent,  above  synchronous 
speed,  the  machine  will  be  delivering  full-load  as  a  generator. 

319.  Self -starting  Synchronous  Motors. — Polyphase  syn- 
chronous motors  have  stators  which  are  exactly  the  same  as  those 
of  induction  motors,  they  may  therefore  be  made  self  starting  if  a 
squirrel  cage  winding  is  added  to  the  rotor  as  shown  in  Fig.  348. 


POLYPHASE  INDUCTION  MOTORS  295 

If  alternating  e.m.fs.  are  applied  to  the  stator  of  such  a  machine, 
the  field  coils  not  being  excited,  the  machine  will  start  up  as  an 
induction  motor  and  will  attain  practically  synchronous  speed. 
If  the  field  coils  are  then  excited,  the  machine  will  be  pulled  into 
synchronism. 

The  single-phase  synchronous  motor  cannot  be  made  self 
starting  in  this  way  because,  as  shown  on  page  308,  the  single- 
phase  induction  motor  is  not  self  starting. 

320.  Dampers  for  Synchronous  Machines. — In  addition  to 
making  a  synchronous  machine  self  starting,  the  squirrel  cage 
acts  as  a  damper  to  prevent  hunting,  see  page  259. 

A  machine  which  is  hunting  is  moving  at  a  speed  which  is 
alternately  slower  and  faster  than  the  synchronous  speed,  and, 
in  each  case,  the  squirrel  cage  rotor  cuts  lines  of  force.  A  driving 
torque  is  produced  as  in  an  induction  motor  when  the  machine  is 
running  below  synchronous  speed,  and  this  tends  to  speed  it  up, 
while  a  retarding  torque  is  produced  as  in  an  induction  generator 
when  the  machine  is  running  above  synchronous  speed,  and  this 
tends  to  slow  it  down.  When  the  machine  is  running  at  syn- 
chronous speed  there  is  no  current  in  the  squirrel  cage.  The  tor- 
que due  to  the  squirrel  cage  therefore  tends  at  all  times  to  pre- 
vent oscillations  in  speed -and  therefore  to  prevent  hunting. 


CHAPTER   XXXVII 
INDUCTION  MOTOR  APPLICATIONS  AND  CONTROL 

321.  Choice  of  Type  of  Motor. — The  synchronous  motor  is 
the  only  motor  whose  power  factor  can  be  controlled.  Such  a 
machine  may  be  overexcited  and  made  to  draw  a  leading  current 
from  the  line  so  as  to  raise  the  average  power  factor  of  the  total 
load  connected  to  the  line  and  thereby  allow  the  use  of  a  smaller 
alternator  and  a  smaller  cross  section  of  copper  in  the  transmis- 
sion line  than  would  be  required  for  a  load  consisting  entirely  of 
induction  motors,  see  page  257. 

The  synchronous  motor  runs  at  a  constant  speed  at  all  loads. 
It  suffers  from  two  disadvantages  namely,  that  the  starting 
torque  is  small  even  if  the  motor  is  of  the  self-starting  type,  while 
direct  current  is  necessary  to  excite  the  field  magnets.  Because 
of  these  disadvantages  it  is  used  only  in  large  sizes  and  only  when 
the  starting  torque  required  is  small  as,  for  example,  for  the 
driving  of  large  constant  speed  fans,  pumps,  and  air  compressors, 
when  the  load  can  be  relieved  during  the  starting  period;  it  is 
also  much  used  as  the  motor  end  of  a  motor  generator  set,  see 
page  318. 

The  single -phase  machines  described  in  the  next  chapter  are 
more  expensive  than  polyphase  machines  of  the  same  output  and 
are  not  used  for  general  power  work.  They  are  used  in  compara- 
tively small  sizes  where  polyphase  current  is  not  available. 

The  polyphase  induction  motor  is  used  for  practically  all  the 
power  work  when  only  alternating  current  is  available,  care  must 
be  taken  however  to  use  the  proper  type. 

The  squirrel-cage  induction  motor  takes  a  lagging  current  and 
has  a  full-load  power  factor  of  about  80  per  cent,  for  a  1  h.p. 
motor  and  90  per  cent,  for  a  100  h.p.  machine. 

It  is  available  only  with  the  following  speeds 

296 


ART.  324]  INDUCTION  MOTOR  APPLICATIONS  297 

Poles  On  25-cycle  On  60-cycle 

mains  mains 

2  1500r.p.m.  3600  r.p.m. 

4  750  r.p.m.  1800  r.p.m. 

6  500  r.p.m.  1200  r.p.m. 

8  375  r.p.m.  900  r.p.m. 

10  300  r.p.m.  720  r.p.m. 

p  120,  X  25  r.p.m.  120  X  60  r.p.m. 

P  P 

The  full-load  speed  is  about  4  per  cent,  lower  than  the  above. 

The  principal  objection  to  the  squirrel-cage  motor  is  that  it  takes 
a  large  starting  current  to  develop  a  moderate  starting  torque. 
About  5  times  full-load  current  is  required  to  give  1.5  times  full- 
load  torque  or,  with  reduced  voltage,  see  page  302,  3.3  times  full- 
load  current  for  full-load  torque.  The  motor  is  therefore  not 
suitable  for  heavy  starting  duty. 

It  is  very  rugged,  has  no  sliding  contacts,  and  is  the  most 
satisfactory  constant  speed  motor  for  loads  that  do  not  require  a 
large  starting  torque. 

The  wound  rotor  induction  motor  has  the  same  running  char- 
acteristics as  the  squirrel-cage  motor,  but  has  better  starting 
characteristics  and  will  develop  full-load  torque  at  starting  with 
full-load  current  in  the  line. 

The  service  for  which  each  of  these  induction  motors  is  suited 
can  best  be  illustrated  by  a  few  typical  examples. 

322.  A  line  shaft  should  run  at  practically  constant  speed  at  all 
loads  and  may  be  driven  by  a  squirrel-cage  motor  if  there  are  not 
many  countershaft  .belts,  or  if  loose  pulleys  are  used  to  take  the 
load  off  the  motor  during  the  starting  period.     If  the  starting 
torque  required  exceeds  full-load  torque  then  a  wound  rotor  type 
of  motor  must  be  used. 

323.  Wood-working  machinery  such  as  planers  and  saws  are 
driven  by  squirrel-cage  motors,  except  in  the  case  of  certain  types 
of  heavy  planing  mills  which  have  a  large  inertia  and  require  the 
large  starting  torque  developed  by  the  wound  rotor  motor. 

324.  Cement  Mills. — The  squirrel- cage  motor  is  used  for  the 
driving  of  cement  machinery  because  it  has  no  sliding  contacts; 
the  commutator  of  a  direct-current  motor  or  the  slip  rings  of  a 
wound  rotor  motor  would  wear  very  rapidly  in  a  cement  mill. 

To  obtain  the  high  starting  torque  required  for  some  of  the 
machines,  a  large  resistance  must  be  inserted  in  the  rotor  circuit, 
see  page  289.  In  the  case  of  the  squirrel  cage  motor  this  is  done 


298    PRINCIPLES  OF  ELECTRICAL  ENGINEERING     [CHAP,  xxxvii 

by  making  the  rotor  end  connectors  of  high  resistance  metal. 
The  resistance  in  this  case  remains  permanently  in  the  circuit 
and  causes  the  efficiency  at  full-load  to  be  about  5  per  cent,  lower 
than  it  would  be  with  a  wound  rotor  motor,  since  this  latter  ma- 
chine is  so  constructed  that  the  resistance  can  be  cut  out  when  the 
motor  is  up  to  speed. 

325.  Motors    for   Textile    Machinery.— The    quality    of    the 
finished  product  of  a  loom  depends  largely  on   the  constancy 
of  the  motor  speed  and  for  such  service  induction  motors  are 
preferred  to  direct- current  motors  because  their  speed  depends 
only  on  the  frequency  of  the  power  supply  and  is  not  affected  by 
variations  of  the  applied  voltage. 

326.  Adjustable   Speed   Motors. — A   wound   rotor  induction 
motor  with  the  rotor  short-circuited  drops  about  4  per  cent,  in 


s 

Stator  Switch 


100 


*•" 


2    40 

"3 

a 
o  20 

I 


25         50         75        100      125      150 
PCJ:  Cent  of  Full  Load  ToKjue 

FIG.  349.  FIG.  350. 

Effect  of  rotor  resistance  on  the  speed  of  an-  induction  motor. 

speed  between  no-load  and  full-load,  as  shown  in  curve  a,  Fig.  350. 

When  resistance  is  inserted  in  the  rotor  circuit,  as  shown  in 
Fig.  349,  the  motor  will  drop  in  speed  so  as  to  generate  the  larger 
e.m.f.  required  to  send  the  current  through  this  higher  resistance. 
At  no-load  however  the  rotor  will  run  at  practically  synchronous 
speed  because  only  a  small  slip  is  required  to  generate  enough 
e.m.f.  to  send  the  no-load  current  through  the  rotor.  Curves 
b  and  c,  Fig.  350,  are  the  speed  curves  with  different  values  of 
rotor  resistance;  the  larger  the  drop  in  speed  required,  the  larger 
the  rotor  resistance  must  be,  and  the  greater  the  loss  in  this 
resistance. 

327.  Crane  motors  should  be  able  to  develop  a  large  starting 
torque  without  taking  an  excessive  current  from  the  line;  they 


ART.  329]  INDUCTION  MOTOR  APPLICATIONS  299 

should  run  at  a  slow  speed  when  the  load  to  be  lifted  is  heavy 
and  at  a  high  speed  when  the  load  is  light. 

Large  starting  torque  may  be  obtained  by  inserting  resistance 
in  the  rotor  circuit.  By  this  means  full-load  torque  may  be 
obtained  at  starting  with  full-load  current  in  the  line,  and  twice 
full-load  starting  torque  with  about  twice  full-load  current,  see 
page  289. 

A  drooping  speed  characteristic  such  as  that  in  curve  c,  Fig.  350, 
is  also  obtained  by  inserting  resistance  in  the  rotor  circuit. 

Crane  motors  in  small  sizes  are  generally  of  the  squirrel  cage 
type,  the  high  rotor  resistance  being  obtained  by  making  the  rotor 
end  connectors  of  high  resistance  metal.  This  gives  a  very  simple 
type  of  machine  but,  because  of  the  large  rotor  resistance  and 
therefore  the  large  rotor  resistance  loss,  the  machine  heats  up 
quickly.  For  outputs  greater  than  20  h.p.  it  is  therefore  advisable 
to  use  the  wound  rotor  type  of  machine  in  which  case  the 
resistance  is  outside  of  the  machine  and  can  be  varied  as  desired. 

Crane  operation  by  means  of  induction  motors  is  not  so  efficient 
as  the  operation  by  direct-current  series  motors,  since  these  latter 
machines  have  the  desired  characteristics  without  the  use  of 
additional  resistance,  see  page  103. 

328.  Shears  and  punch  presses  are  generally  supplied  with  a 
flywheel   to   carry  the  peak  load.     In  order  that  the  flywheel 
may  be  effective,  the  speed  of  the  motor  must  drop  as  the  load 
comes  on.     A  standard  machine  drops  in  speed  about  4  per  cent, 
between  no-load    and   full-load.     For  a   larger  drop  in  speed, 
resistance  must  be  connected  permanently  in  the  rotor  circuit 
so  as  to  give  a  characteristic  such  as  that  shown  in  curve  6,  Fig. 
350.     The   motor   may  be  a  squirrel-cage  machine  with  high 
resistance  end  connectors,  but  for  large  presses  it  will  often  be 
necessary  to  use  a  wound  rotor  motor  to  obtain  sufficient  starting 
torque  to  accelerate  the  flywheel  on  first  starting  up. 

329.  For  adjustable  speed  service  such  as  the  driving  of  lathes 
and  other  machine  tools,  the  only  alternating-current  motor  at 
present  available  is  the  wound  rotor  induction  motor. 

The  speed  of  such  a  machine  may  be  lowered  for  a  given  torque 
by  inserting  resistance  in  the  rotor  circuit  as  shown  in  Fig.  349, 
and  the  speed  characteristics  with  different  values  of  resistance 
are  shown  in  Fig.  350. 

Because  of  the  use  of  this  resistance,  there  is  a  large  resistance 
loss  which  increases  as  the  speed  is  decreased,  so  that  the  efficiency 


300     PRINCIPLES  OF  ELECTRICAL  ENGINEERING   [CHAP,  xxxvn 

.of  an  induction  motor  at  reduced  speed  is  low.  Moreover  the 
speed  regulation  is  poor,  as  shown  by  curves  6  and  c,  Fig.  350. 
If  for  example  an  induction  motor  operating  at  reduced  speed  is 
driving  a  lathe  in  which  a  forging  such  as  that  in  Fig.  129,  page  108, 
is  being  turned,  then  the  motor  will  slow  down  when  the  cut  is 
deep  and  will  speed  up  when  the  cut  is  light,  so  that  the  speed 
will  be  very  irregular. 

For  machine  tool  driving  then  the  alternating-current  motor 
is  not  such  a  suitable  machine  as  the  direct- current  shunt  motor 
controlled  by  field  resistance,  see  Arts  126  and  127,  page  107, 
and  it  has  been  pointed  out  that  for  crane  service  the  direct- 
current-  motor  is  superior  to  the  alternating- current  motor,  so 
that,  when  the  bulk  of  the  load  consists  of  adjustable  speed  or 
crane  motors,  direct  current  should  be  supplied  even  if  it  is  neces- 
sary to  use  a  motor  generator  set,  see  page  318,  to  change  from 
alternating  to  direct  current. 

330.  Resistance  for  Adjustable  Speed  Motors. — As  in  the  case 
of  the  direct-current  motor,  the  number  of  ohms  in  the  controlling 
rheostat  and  the  current  carrying  capacity  of  the  resistors  de- 
pends on  the  service  for  which  the  motor  has  to  be  used. 

If  a  fan  and  a  reciprocating  pump  take  10  h.p.  at  full 
speed,  then  at  half  speed  the  pump  takes  5  h.p.  since  the 
torque  is  constant,  while  the  fan  takes-  ^10  =  2.15  h.p..  see 
page  111,  the  rotor  current  of  the  fan  motor  will  therefore 
be  less  at  half  speed  than  that  of  the  pump  motor.  The  voltage 
generated  in  the  rotor  by  the  revolving  field,  however,  will  be  the 
same  in  each  case,  so  that  the  number  of  ohms  in  the  fan  rheostat 
must  be  greater  than  the  number  in  the  pump  rheostat  so  as  to 
reduce  the  current  to  the  lower  value. 


STARTERS  AND  CONTROLLERS 

331.  Switches  for  Alternating -current  Circuits. — It  causes 
less  arcing  to  open  a  circuit  in  which  alternating  current  is  flowing 
than  to  open  one  in  which  direct- current  is  flowing,  because,  in 
the  former  case,  the  current  passes  through  zero  twice  every  cycle. 

The  quick  break  type  of  switch,  see  page  114,  is  used  for  small 
currents.  For  larger  currents  or  in  high-voltage  circuits  the 
contacts  are  immersed  in  insulating  oil  which  quenches  any  arc 
that  forms.  The  oil  switch  in  alternating-current  circuits  takes 


ART.  332] 


INDUCTION  MOTOR  APPLICATIONS 


301 


the  place  of  the  magnetic  blow-out  switches  used  in  direct-current 
circuits. 

332.  Starting  of  Squirrel-cage  Induction  Motors. — A  squirrel 
cage  motor  at  standstill  takes  about  5  times  full-load  current 
from  the  line  with  normal  applied  voltage,  and  develops  about 
1.5  times  full-load  torque. 


Running  Positio 
\ 

Starting  Position 


FIG.  351. — Connections  for  a  small  three-phase,   squirrel-cage  induction 

motor. 

Motors  with  an  output  of  5  h.p.  or  less  are  generally 
connected  directly  to  the  line  without  a  starter,  by  means  of  a 
double-throw  switch  such  as  that  shown  diagrammatically  in 
Fig.  351.  This  switch  is  so  constructed  that  it  will  stay  in  the 
starting  position  only  when  held  there  against  the  tension  of  a 
spring. 


Auto  Transformers 

FIG.  352. — Connections    of    a    starting   compensator  for   a    three-phase, 
squirrel-cage  induction  motor. 

Motors  with  an  output  which  is  greater  than  5  h.p.  are 
generally  started  up  on  reduced  voltage  by  means  of  autotrans- 
formers  connected  as  shown  in  Fig.  352.  By  this  means  the  start- 
ing current  is  reduced,  but  the  starting  torque  also  is  reduced; 
the  revolving  field  $  is  proportional  to  the  applied  voltage  E  which 
produces  it,  see  page  292,  while  the  rotor  current  72  is  proportional 
to  the  revolving  field  <£  by  which  it  is  produced,  so  that  the  start- 


302  PRINCIPLES  OF  ELECTRICAL  ENGINEERING  [CHAP,  xxxvn 

ing  torque,  being  proportional  to  <£  X  /2  is  proportional  to   <£2 
and  therefore  to  E2. 

A  motor  at  standstill  takes  5  times  full-load  current  with  normal  applied 
voltage  and  develops  1.5  times  full-load  torque.  -  What  must  the  applied  volt- 
age be  to  obtain  full-load  torque  and  what  will  be  the  starting  currents  in  the 
motor  winding  and  in  the  line  ? 

full-load  torque  J_ 

1.5  full-load  torque    ~  1.5 


therefore  E2  =      /  —  X 

.5 


•sS 


or  81.5  per  cent,  of  normal  voltage 

E2 

The  starting  current  in  the  motor  =  5  (full-load  current)  _ 

EI 

=  4.1  (full-load  current) 
=  Im,  Fig.  352. 

ET 

The  starting  current  in  the  line  =4.1  (full-load  current)  — 

EI 

=  3.3  (full-  load  current) 
=  Ii,  Fig.  352. 

333.  Starting  Compensator.  —  The  combined  autotransformers 
and  switches  constitute  what  is  called  a  starting  compensator. 
One  type  is  shown  in  Fig.  353  and  consists  essentially  of  three 
autotransformers  T  and  a  double-throw  switch  S  by  means  of 
which  the  motor  is  connected  to  low-  voltage  taps  for  starting  and 
is  then  connected  directly  to  the  line  when  nearly  up  to  full  speed. 

The  complete  connections  of  this  compensator  are  shown  in 
diagram  A,  Fig.  354. 

Diagram  B  shows  the  connections  during  the  starting  period; 
normal  voltage  EI  is  applied  to  the  lines  a,  b,  and  c  while  a  reduced 
voltage  Ez  is  tapped  off  from  the  autotransformers  and  is  applied 
to  the  motor. 

Diagram  C  shows  the  connections  during  the  running  period. 
The  voltage  applied  to  the  motor  is  now  normal  and  the  overload 
relays  0  are  connected  in  the  circuit  while  the  no-  voltage  release 
coil  M  is  connected  across  one  leg  of  the  circuit. 

The  no-voltage  release  feature  is  similar  in  principle  to  that 
used  on  starters  for  direct-current  motors,  see  page  120,  and 
consists  of  a  latching  solenoid  which  holds  the  starting  arm  against 
the  tension  of  a  spring.  When  the  line  voltage  fails,  the  solenoid 


ART.  333]  INDUCTION  MOTOR  APPLICATIONS 


303 


No  Voffaq 

Release 


Oil  tcrn/f 


FIG.  353. — Starting  compensator  for  a  three-phase  induction  motor. 


From  Generator 


Auto  Transformers 


A  B  C 

Complete  connections  Starting  Running 

FIG.  354. — Diagram  of  connections  of  a  three-phase  starting  compensator. 


304  PRINCIPLES  OF  ELECTRICAL  ENGINEERING    [CHAP,  xxxvn 

M  is  deenergized  and  the  starting  handle  then  returns  to  the 
off  position. 

In  the  case  of  a  heavy  overload,  the  plungers  of  the  overload 
relays  0  are  raised  and  open  the  circuit  of  the  no- voltage  release 
M,  the  starting  handle  then  returns  to  the  off  position. 

The  no- voltage  release  is  not  supplied  in  all  cases  because  an 
induction  motor  can  carry  a  starting  current  of  5  times  full-load 
current  without  injury  during  the  short  interval  of  time  taken  to 
attain  full  speed.  As  a  rule,  the  only  objection  to  this  large 
current  is  the  disturbance  it  causes  due  to  the  opening  of  circuit 
breakers. 

334.  The  star-delta  method  of  starting  is  sometimes  used  for 
three-phase  motors.  The  windings  of  the  three  phases  are  kept 


Jo  at  Staudstill 


at  Standstill 


at  Standstill  Vo 

l-Starting  Connection  B- Running  Connection 

FIG.  355. — Voltage  and  current  relations  in  a  Y-delta  starter. 

separate  from  one  another  and  six  leads  are  brought  out  from  the 
machine.  Under  normal  running  conditions  the  windings  are 
delta  connected  as  shown  in  diagram  B,  Fig.  355,  and  the  voltage 
per  phase  is  E\  volts.  During  the  starting  period  the  windings 
are  Y- connected  as  shown  in  diagram  A  in  which  case  the  voltage 
per  phase  is  equal  to  #1/1.73  or  58  per  cent,  of  normal  voltage. 

If  a  delta-connected  motor  at  standstill  takes  5  times  full-load  current  with 
normal  applied  voltage  and  develops  1.5  times  full-load  torque,  what  is  the 
starting  current  in  the  motor  and  also  in  the  line,  if  the  motor  is  Y-connected, 
and  what  is  the  starting  torque  under  these  conditions  ?  (The  student  should 
make  a  diagram  of  connections  showing  the  double-throw  switch  required  to 
change  from  Y  to  delta.) 

The  starting  torque  =  1.5  (full-load  torque)  X  (0.58)2 
=  0.5  (full-load  torque) 


ART.  335] 


INDUCTION  MOTOR  APPLICATIONS 


305 


The  starting  current  in  the  line  when  delta  connected       =  Is 

The  starting  current  in  the  motor  when  delta  connected  =  /»/\/3 

The  starting  current  in  the  motor  when  Y-connected         =  (/s/\/3)  X  0.58 

=  L/3 
The  starting  current  in  the  line  when  Y-connected  =  /.,/3 

Now  Is  is  5  times  full-load  current,  so  that,  when  the  motor  is  Y-connected, 
the  starting  current  in  the  line  is  1/3  X  5  or  1.67  times  full-load  current. 
The  starting  torque,  however,  is  only  0.5  times  full-load  torque  and,  if  this 
is  not  sufficient  to  start  the  motor  with  the  load,  then  a  starting  compensator 
will  be  required. 

335.  Starter  for  a  Wound  Rotor  Motor. — To  start  up  a  motor 
of  this  type  the  main  switch  S,  Fig.  349,  is  closed  and  then  the 


FIG.  356. — Sliding  contact  type  of  starter  for  a  wound-rotor  induction  motor . 


resistance  R  in  the  rotor  circuit  is  gradually  cut  out  as  the  motor 
comes  up  to  speed. 

Since  the  rotor  is  wound  in  three  sections,  see  page  290,  three 
sets  of  contacts  are  required  which  for  small  motors  are  generally 
mounted  on  a  faceplate  as  shown  in  Fig.  356.  This  starter  may 
be  used  as  a  speed  regulator  if  the  resistance  has  sufficient 
current  carrying  capacity. 

For  motors  larger  than  about  50  h.p.,  the- multiple  switch 
type  of  starter,  see  page  119,  is  generally  preferred.  Such  a 
starter  for  the  motor  shown  diagrammatically  in  Fig.  357  would 
consist  of  three  double-pole  switches  so  interlocked  that  they  can 
be  closed  only  in  the  order  A,  B,  C.  The  switches  are  held  closed 


20 


306   PRINCIPLES  OF  ELECTRICAL  ENGINEERING   [CHAP,  xxxvn 


by  a  latch  which  is  released  by  the  no-voltage  release  magnet  when 
the  voltage  fails. 

336.  Automatic  Starters. — Squirrel-cage  motors  of  small  out- 
put are  thrown  directly  on  the  line  and  the  self  starter  for  such  a 
motor  consists  of  a  single  double-pole  magnetic  switch  such  as 
that  shown  diagrammatically  at  A,  Fig.  357. 

If  a  solenoid  were  attached  to  the  handle  of  a  starting  rheostat 
such  as  that  in  Fig.  356,  then  a  sliding  contact  type  of  self  starter 


FIG.  357. — Wound  rotor  type  of  motor. 


a  4       C 


FIG.  359. 


FIG.  358. — Automatic  starter  for  a  wound-rotor  in- 
duction motor. 


would  be  produced  similar  to  the  direct-current  starter  shown 
in  Fig.  154,  page  130.  If  in  addition  the  main  starter  switch  is 
magnetically  operated  then  the  motor  may  be  started  from  a 
distance  by  a  control  circuit  such  as  that  shown  in  Fig.  156,  page 
131,  this  circuit  being  connected  across  one  of  the  phases  and  the 
magnets  being  laminated1  so  as  to  be  suitable  for  operation  with 
alternating  currents. 

1  The  core  of  an  alternating- current  magnet  must  be  laminated  for  the  same 
reason  as  a  transformer  core  is  laminated,  see  page  269,  namely,  to  prevent 
excessive  core  loss  due  to  the  alternating  magnetic  flux. 


ART.  336.]          INDUCTION  MOTOR  APPLICATIONS  307 

For  large  motors,  the  multiple  switch  type  of  starter  is  used, 
by  means  of  which  the  switches  A,  B  and  C,  Fig.  357  are  closed 
in  their  proper  order  and  the  motor  thereby  brought  up  to  speed 
without  the  starting  current  exceeding  a  predetermined  value. 
The  principle  of  operation  is  the  same  as  for  the  direct-current 
starter  described  in  Art.  158,  page  133,  although  the  mechanical 
details  are  different. 

Fig.  358  shows  the  control  circuit  for  the  three  switches  A,  B 
and  C  in  Fig.  357.  When  the  control  switch  s  is  closed,  the  line 
b  is  excited  as  far  as  point  c,  and  the  magnet  X,  connected  between 
points  1  and  2,  closes  the  double  contactor  switch  A.  The 
motor  then  starts  up  with  all  the  rotor  resistance  R,  Fig.  357, 
in  the  circuit,  while  about  one  and  a  half  times  full-load  current 
flows  in  the  lines. 

When  switch  A  closes,  the  disc  d  drops,  closes  the  contacts  e  and 
/,  and  extends  the  excited  part  of  line  b  as  far  as  g.  The  coil  t  is 
now  excited  from  the  points  3  and  4  and  its  plunger  is  lifted, 
thereby  tilting  the  lever  j  to  which  it  is  attached  and  removing 
the  support  of  the  plunger  of  coil  u.  The  line  current  passing 
around  u  however  holds  up  this  plunger  until  the  motor  has 
attained  about  one-third  of  normal  speed  and  the  current  has 
decreased  to  about  full-load  value  when  the  plunger  of  u  drops 
and  closes  the  contacts  h  and  k.  The  coil  Y  is  now  excited  from 
the  points  5  and  6  and  closes  the  double-pole  switch  B  thereby 
cutting  out  the  first  step  of  the  resistance  from: -each  leg  of  the 
rheostat.  The  current  then  increases  to  about  1.5  times  full-load 
current  but  gradually  decreases  as  the  motor  speeds  up  further. 

As  soon  as  switch  B  closes,  the  interlock  discs  attached  to  that 
switch  are  lowered,  and  the  contact  Im  is  closed,  so  that  coil  Y  is 
now  excited  from  the  points  5  and  7  and  the  switch  B  is  therefore 
held  closed  independently  of  the  plunger  of  coil  u.  The  contact 
np  is  closed  at  the  same  instant  by  the  lower  interlock  disc  and 
the  excited  part  of  line  b  is  thereby  extended  as  far  as  q,  and  the 
same  series  of  operations  is  repeated  before  switch  C  closes. 

Fig.  359  shows  the  complete  starter,  with  the  relays  t  and  v 
on  the  top  panel  and  the  switches  A,  B  and  C  on  the  lower 
panels. 


CHAPTER  XXXVIII 
SINGLE-PHASE  MOTORS 

337.  Single-phase  Induction  Motors. — If  one  of  the  phases  of  a 
two-phase  induction  motor  is  opened  while  the  motor  is  running, 
the  machine  will  continue  to  rotate  and  carry  the  load. 

A  two-pole  single-phase  induction 
motor  at  standstill  is  shown  diagram- 
matically  in  Fig.  360.  When  an  al- 
ternating current  flows  in  the  winding 
A,  an  alternating  flux  </>  is  produced. 
Since  this  magnetic  field  is  not  rotat- 
ing, there  is  no  tendency  for  the  rotor 
to  turn,  so  that  the  machine  is  not  self 
Starting.  FlG>  360.— Diagrammatic 

338.  Split-phase  Method  of  Starting,   representation  of  a    single- 
~  -    .,  ,1      i  T  .      phase  induction  motor. 

—One  of  the  many  methods  used  to 

obtain  a  rotating  field  from  a  single-phase  supply  is  shown  dia- 
grammatically  in  Fig.  361.  The  motor  is  wound  as  for  two- 
phase  operation,  and  a  resistance  R  is  inserted  in  series  with 
the  winding  A.  The  current  Ia  therefore  does  not  lag  as  much 
as  the  current  /&,  and  the  resultant  magnetic  field  of  the  motor 
has  a  rotating  component  which  will  cause  the  rotor  to  turn. 

If,  in  addition  to  the  resistance  R,  a  condenser  C  is  inserted  in 
series  with  the  winding  A}  then  the  current  Ia  may  be  made  to 
lead  the  voltage  E.  The  currents  Ia  and  7&  will  then  be  more 
nearly  90  degrees  out  of  phase,  and  the  operation  of  the  motor 
will  be  more  nearly  that  of  a  two-phase  machine. 

This  method  of  starting  is  called  the  split-phase  method. 

339.  Running  Torque  of  a  Single-phase  Motor. — If,   as  in 
Fig.  363,  two  equal  vectors  P  rotate  with  the  same  velocity  but  in 
opposite  directions,  the  resultant  vector  R  alternates  between  the 
values  R  =  2P  and  R  =  —2P,  and  always  lies  in  the  line  joining 
these  two  points. 

An  alternating  magnetic  flux  0,  such  as  that  in  Fig.  362,  is 
therefore  equivalent  to  two  rotating  fields  <f>x  and  <j>y  which  are  of 

308 


ART.  339] 


SINGLE-PHASE  MOTORS 


309 


equal  strength  and  rotate  in  opposite  directions  with  the  same 
velocity.  These  fields  tend  to  start  the  rotor  in  opposite  direc- 
tions, so  that  the  resultant  starting  torque  is  zero. 


E 


Resistance  in  series  with  ona         Resistance  and  condenser   in 
winding  series  with  one  winding 

FIG.  361. — Split-phase   connections  for   a   single-phase   induction   motor. 


FIG.  362.  . 

FIGS.  362  and  363. — Resolution  of  an  alternating  field  into  two  revolving 

components. 

Suppose  that  the  rotor  has  been  started  by  phase-splitting  and 
is  running  in  the  direction  of  the  field  <f>x  with  a  speed  which  is 
little  less  than  synchronous  speed.  The  rotor  bars  will  then  be 
cutting  the  field  <f>x,  and  an  e.m.f.  will  be  induced  which  will  send 


310  PRINCIPLES  OF  ELECTRICAL  ENGINEERING  [CHAP,  xxxvm 

through  these  bars  a  current  Ix  of  frequency  sf,  see  page  291, 
where  s  is  the  per  cent,  slip  and/  is  the  applied  frequency.  Since 
this  rotor  frequency  is  low,  the  current  lags  by  a  very  small  angle, 
and  the  torque  due  to  this  current  is  large,  see  page  288. 

The  rotor  bars,  however,  are  also  cutting  the  field  0y;  and, 
since  this  field  is  moving  in  a  direction  opposite  to  that  of  the 
rotor,  the  frequency  of  the  resulting  rotor  current  Iv  is  almost 
equal  to  2/.  This  frequency  is  high,  and  the  current  Iy  lags  by 
a  large  angle,  so  that  the  torque  due  to  this  current  is  negligible, 
see  page  288. 

The  resultant  rotor  current  is  the  resultant  of  Ix  and  Iy,  but 
the  latter  current  is  not  effective  in  producing  torque,  and  the 

Armature  Flux  (f>  a 

-5 

7 


FIG.  364.  FIG.  365. 

FIGS.  364  and  365. — Single-phase  series  motor. 

machine  runs  as  if  it  had  only  one  field  4>x.  The  characteristics 
with  respect  to  slip,  efficiency  and  power  factor  are  therefore 
similar  to  those  of  a  polyphase  induction  motor. 

340.  Single-phase  Series  Motor. — This  machine  is  wound  and 
connected  like  a  direct-current  series  motor,  but  a  few  structural 
changes  are  necessary  to  make  a  machine  which  will  operate  with 
alternating  current. 

In  Fig.  364,  the  current  is  flowing  through  such  a  machine  in 
one  direction,  while  in  Fig.  365,  half  a  cycle  later,  the  current 
is  flowing  in  the  opposite  direction  in  both  armature  and  field 
coils,  but  the  direction  of  the  torque  is  unchanged. 

The  characteristics  of  this  machine  are  similar  to  those  of  the 
direct-current  series  motor.  The  torque  is  approximately  pro- 
portional to  the  square  of  the  current,  since  torque  =  K<t>I, 
where  the  flux  <f>  is  proportional  to  the  current  7.  The  torque, 
however,  is  pulsating,  being  a  maximum  when  the  current  is  a 
maximum  and  zero  when  the  current  is  zero,  but  the  average 


ART.  340] 


SINGLE-PHASE  MOTORS 


311 


torque  is  the  same  as  would  be  produced  by  a  direct  current  of 
the  same  magnitude. 

Since  the  magnetic  flux  in  the  poles  is  alternating,  the  field 
structure  must  be  laminated  to  keep  the  eddy  current  loss  small. 
Due  to  this  alternating  flux,  a  voltage  E/  of  self  induction  is 
induced  in  the  field  coils,  to  overcome  which  the  applied  voltage 
must  have  a  component  Ef,  Fig.  366. 


Motor  Current 


FIG.  366. — Vector  diagram  for  a  single-phase  series  motor. 

Since  the  armature  is  rotating  in  a  magnetic  field,  a  back  e.m.f. 
is  generated  in  the  armature  in  the  same  way  as  in  a  direct-current 
motor,  see  page  79.  This  e.m.f.,  however,  is  alternating  since 
it  is  produced  by  the  cutting  of  an  alternating  flux  </>.  It  is  a 
maximum  when  the  flux  is  a  maximum  and  zero  when  the  flux  is 
zero,  so  that,  to  overcome  this  back  e.m.f.,  the  applied  voltage 


25  50          75         100        125 

Per  Cent  of  Full  Load  Current 

FIG.  367. — Characteristics  of  a  single-phase  series  motor. 

must  have  a  component  Ea  in  phase  with  the  flux  0  and  therefore 
in  phase  with  the  current  7. 

In  Fig.  366,  then 

<f>  is  the  alternating  magnetic  flux 

/  is  the  current  in  the  machine 

Ea  is  the  component  of  voltage  to  overcome  the  back  e.m.f. 


312  PRINCIPLES  OF  ELECTRICAL  ENGINEERING  [CHAP,  xxxvm 


Ef  is  the  component  of  e.m.f.  to  send  the  current  I  through  the 
self  induction  of  the  field  coils 

E,  the  resultant  of  Ea  and  Ef)  is  the  applied  voltage 

cos  a  is  the  power  factor  of  the  motor. " 

The  lower  the  frequency  of  the  supply,  the  smaller  the  value 
of  Ef't  and  the  higher  the  speed  of  the  motor,  the  larger  the 
value  of  Ea,  so  that  low  frequency  and  high  speed  are  the  condi- 
tions for  high  power  factor. 

If  the  load  on  such  a  motor  is  increased,  the  current  increases 
and  so  therefore  does  the  flux  </>  and  the  voltage  Ef.  Since  the 


FIG.  368. — Conductively  compen-          FIG.  369. — Inductively  compen- 
sated, sated. 

FIGS.  368  and  369. — Methods  of  compensating  for  armature  reaction  in  a 
single-phase  series  motor.  . 

applied  voltage  E  is  constant,  therefore  Ea  decreases  slightly;  to 
generate  this  back  e.m.f.  with  the  larger  flux  <£,  the  armature 
must  run  at  a  slower  speed.  Typical  curves  for  such  a  motor 
are  shown  in  Fig.  367.  The  speed  and  torque  curves  are  similar 
to  those  of  a  direct-current  series  motor,  and  such  a  machine  will 
run  equally  well  with  either  a  direct-  or  an  alternating-current 
supply. 

341.  Armature  Reaction. — From  Figs.  364  and  365  it  may  be 
seen  that  there  is  a  cross  magnetizing  effect  due  to  the  armature 
current  just  as  in  the  direct- current  machine,  but  this  cross  flux 
(f>a  is  alternating  and  generates  an  e.m.f.  of  self  induction  in  the 
armature  coils  which  tends  to  lower  the  power  factor,  because 
self  induction  always  tends  to  make  the  current  lag.  This  cross 
flux  is  eliminated  by  means  of  a  compensating  winding  connected 
as  shown  in  Fig.  368  so  that  its  magnetizing  effect  at  every  instant 
is  exactly  equal  and  opposite  to  that  of  the  armature. 


ART.  342J 


SINGLE-PHASE  MOTORS 


313 


Another  method  of  eliminating  the  cross  flux  '  <j>a  is  shown 
diagrammatically  in  Fig.  369.  In  this  case  the  compensating 
coils  are  short  circuited.  The  flux  0a  threads  these  coils  and 
generates  in  them  an  e.m.f.  which,  according  to  Lenz's  law,  sends 
a  current  in  such  a  direction  as  to  oppose  the  change  of  the  flux, 
so  that  the  cross  flux  is  automatically  kept  down  to  a  small  value. 

342.  The  Repulsion  Motor. — If,  in  the  machine  shown  in  Fig. 
368,  the  armature  is  disconnected  from  the  circuit  and  is  then 
short  circuited,  as  shown  in  Fig.  370,  the  resulting  machine  is 
what  is  called  the  repulsion  motor. 


A 

FIG.  371.  FIG.  372. 

FIGS.  370  to  372. — The  starting  torque  of  a  single-phase  repulsion  motor. 

If  the  coils  B  alone  are  acting,  as  shown  in  Fig.  371,  then 
e.m.fs.  are  generated  in  the  armature  coils  in  such  a  direction 
as  to  oppose  the  change  of  the  flux  </>,  but  no  current  passes 
through  the  armature  winding  or  through  the  conductor  x  since 
the  voltage  between  a  and  b  is  opposed  by  that  between  b  and  c. 

If  the  coils  A  alone  are  acting,  as  shown  in  Fig.  372,  then  a 


314  PRINCIPLES  OF  ELECTRICAL  ENGINEERING    [CHAP,  xxxvm 

large  current  can  flow,  but  the  resultant  torque  is  zero  because, 
under  each  pole,  half  of  the  conductors  carry  current  in  one  direc- 
tion while  the  other  half  carry  current  in  the  opposite  direction. 
If  both  sets  of  coils  are  acting,  as  in  Fig.  37G,  then  the  current 
produced  in  the  armature  by  coils  A  can  produce  a  torque  with 
the  magnetic  field  produced  by  coils  B. 

343.  Commutation   of   Series   and   Repulsion   Motors. — The 
coils  which  are  shorfc  circuited  by  the  brushes  on  the  commutator 
are  threaded  by  an  alternating  flux  <£,  see  Figs.  364  and  370,  so 
that  large   currents  are  induced  in  these  coils  which  currents, 
flowing  through  the  brush  contacts,  cause  sparking.     The  com- 
mutation of  these  machines  is  therefore  much  poorer  than  that  of 
the  direct-current  series  motor. 

344.  Wagner    Single-phase    Motor. — Because    of    the    poor 
commutation,  the  single-phase  repulsion  motor  has  not  come  into 
extensive  use  in  this  country  except  in  small  sizes.    The  principle 
of  the  repulsion  motor,  however,  is  used  by  the  Wagner  com- 
pany to  obtain  a  large  starting  torque  from  a  single-phase  in- 
duction motor. 

This  machine  starts  up  as  a  repulsion  motor  and,  when  it  has 
attained  almost  synchronous  speed,  a  centrifugal  device  short 
circuits  all  the  commutator  bars  on  one  another  and  at  the  same 
time  lifts  the  brushes.  The  motor  operates  thereafter  as  a  single- 
phase  induction  motor  and  the  commutator  has  a  long  life  because 
it  is  used  only  during  the  starting  period. 


CHAPTER  XXXIX 
MOTOR-GENERATOR  SETS  AND  ROTARY  CONVERTERS 

345.  Motor-Generator  Set. — When  it  is  desired  to  change  from 
one  alternating  voltage  to  another  a  static  transformer  is  used, 
see  Chap.  34.     To  change  from  one  direct  voltage  to  another 
no  such  simple  device  is  available  and  a  motor-generator  set  has 
to  be  used.     This  consists  of  two  machines  mechanically  con- 
nected together  one  of  which,  running  as  a  motor,  takes  power 
from  the  source  of  supply  and  drives  the  other  machine  as  a  gen- 
erator; this  latter  machine  is  wound  for  the  desired  voltage. 

If  10  kw.  at  220  volts  is  required  from  a  110  volt  line  then : 
the  generator  output  =  10  kw.  at  220  volts 

10  v  1000        i 
the  motor    output  = =77; X  Q  ™  =  15  horse-power  at  110  volts 

where  the  generator  efficiency  is  taken  as  88  per  cent. 

346.  The  booster  set  is  a  special  type  of  motor-generator  set 
and  is  shown  diagrammatically  in  Fig.  373.     The  generator  in 


FIG.  373.  —  Booster  motor-generator  set. 

this  case  is  connected  in  series  with  the  line  and  its  voltage  can 
add  to  or  subtract  from  the  line  voltage. 

If  10  kw.  at  220  volts  is  required  from  a  110  volt  line  and  a  booster  set 
is  used  then: 
the  booster  output  =  El,  Fig.373 

=  5  kw.  at  110  volts 

5  X  1000          1 
the  output  of  the  driving  motor  =     —  <TR  —   X 


=  8  horse-power  at  110  volts 

and  this  set  can  perform  the  same  duty  as  that  performed  by  the  motor- 
generator  set  figured  out  in  the  last  article. 

315 


316    PRINCIPLES  OF  ELECT  RICAJL  ENGINEERING    [CHAP.XXXIX 

347.  The  balancer  set  is  a  special  type  of  motor-generator  set 
used  when  it  is  desired  to  change  from  a  two-wire  to  a  three -wire 
direct-current  system.  As  shown  diagrammatically  in  Fig.  374, 
it  consists  of  two  shunt- wound  direct- current  machines  with  their 
armatures  on  the  same  shaft,  which  armatures  in  the  particular 
case  shown,  are  wound  for  110  volts. 

Compare  now  the  operation  of  a  three- wire  system  without  a 
balancer  as  in  Figs.  375  and  377,  and  with  a  balancer  as  in  Figs. 


Balancer 
\ 


FIG.  374. — Balancer  set. 


220 

110 


FIG.  375.    m  FIG.  376. 

FIGS.  375  AND  376. — Balanced  load. 


T  A        >k      TTTTT 

@iioW>  i  i  4  A 

' 


FIG.  377.  FIG.  378.  FIG.  379. 

FIGS.  377  TO  379.— Unbalanced  load. 

376  and  378.  In  the  case  where  the  load  is  equal  on  the  two 
sides  of  the  system,  there  is  no  current  in  the  neutral  wire  and 
the  balancer  in  Fig.  376  is  running  light  as  two  motors  in  series 
on  220  volts,  while  the  voltages  across  the  two  sides  of  the 
system  are  equal. 

If  now  some  of  the  lamps  on  the  side  B  of  the  system  are 
switched  off  then  in  Fig.  377  the  same  total  current  is  passing 
through  the  lamps  connected  across  A  as  is  passing  through  the 
smaller  number  of  lamps  connected  across  B  so  that  each  lamp  B 
is  carrying  more  current  than  each  lamp  A  and  the  voltage  across 
B  is  now  greater  than  that  across  A.  In  the  extreme  case  when 


ART.  348] 


MOTOR-GENERATOR  SETS 


317 


only  one  lamp  is  connected  across  B  the  voltage  across  that  lamp 
is  practically  220  volts. 

When  the  balancer  is  used,  as  in  Fig.  378,  the  voltages  tend  to 
take  the  same  values  as  in  Fig.  377,  but  when  the  voltage  of  B 
rises  and  that  of  A  falls  then  the  machine  M  tries  to  increase  in 
speed  and  G  to  decrease  in  speed  or  each  machine  tries  to  run  as  a 
motor  at  the  speed  corresponding  to  its  voltage.  But  the  two 
machines  are  coupled  together,  so  that  the  actual  speed  must  be 
between  these  two  values  and  the  one  that  tends  to  run  fast  will 
try  to  increase  in  speed  and  will  therefore  act  as  a  motor  and 
drive  the  other  machine  as  a  generator,  so  that  G  operates  as  a 
generator  and  current  passes  through  it  from  the  negative  to  the 
positive  terminal  while  M  operates  as  a  motor  driving  G  and 


FIG.  380. — Three-wire  generator. 

current  passes  through  it  from  the  positive  to  the  negative  termi- 
nal. The  voltage  across  G  will  be  (110  —  7J?a)  volts  and  that 
across  M  will  be  (110  +  IaRa)  volts,  where  IaRa  is  the  armature 
resistance  drop,  and  the  change  in  the  voltages  across  the  two 
sides  of  the  system  is  comparatively  small  even  when  the  load  is 
considerably  unbalanced. 

Neglecting  the  losses  in  the  balancer  itself,  the  generator  out- 
put and  the  motor  input  are  each  equal  to  (110  volts  X  half  the 
unbalanced  current),  and  if  care  is  taken  to  arrange  that  the  load 
is  nearly  balanced  under  all  conditions,  then  the  balancer  may 
be  of  comparatively  small  output. 

When  the  load  on  side  B  of  the  system  becomes  greater  than 
that  on  side  A,  the  current  in  the  neutral  wire  flows  in  the  opposite 
direction,  as  shown  in  Fig.  379. 

348.  Three-wire  Generator. — The  balancer  set  is  eliminated 
by  the  use  of  the  three-wire  generator  shown  diagrammatically 
in  Fig.  380,  which  consists  of  a  standard  two-wire  direct-current 


318    PRINCIPLES  OF  ELECTRICAL  ENGINEERING    [CHAP,  xxxix 

generator  with  a  coil  C  of  high  reactance  and  low  resistance  con- 
nected permanently  across  diametrically  opposite  points  on  the 
armature.  The  voltage  between  a  and  b  is  alternating,  see  page 
240,  so  that,  even  with  no  external  load,  an  alternating  current 
flows  through  the  coil  C,  this  current,  however,  is  extremely 
small  since  the  reactance  of  coil  C  is  large. 

The  center  point  o  is  always  midway  in  potential  between  a 
and  b  and  this  point  is  connected  to  the  neutral  line  of  the  three- 
wire  system.  When  the  loads  on  the  two  sides  of  the  system 
differ,  a  current  flows  in  the  neutral  line  and  enters  the  armature 
through  the  reactance  coil  which  offers  only  a  small  resistance 
to  direct  current.  The  currents  in  the  sections  M  flow  against 
the  generated  e.m.f.  so  that  these  sections  are  equivalent  to  the 
motor  end  of  a  balancer,  while  the  currents  in  sections  G  flow  in 
the  direction  of  the  generated  e.m.f.  and  so  these  sections  are 
equivalent  to  the  generator  end  of  a  balancer. 

349.  To  transform   from  alternating  to   direct  current,   the 
motor-generator  set  may  consist  of   either  an  induction  or   a 
synchronous  motor,  direct  connected  to  a  direct-current  generator, 
The  same  transformation  can  be  made  in  a  single  machine  called 
the  rotary  converter. 

350.  Rotary  Converter. — Fig.  381  shows  an  alternator  such  as 
that  on  page  241  connected  to  a  direct-current  generator  which  has 
the  same  number  of  poles.     If  an  alternating  e.m.f.  is  applied  to 
machine  M,  it  will  operate  as  a  synchronous  motor  and  direct 
current  may  then  be  obtained  from  machine  G.     The  two  ma- 
chines M  and  G  may  be  combined  to  form  a  single  machine  as 
shown  in  Fig.  382,  and,  if  an  alternating  voltage  is  applied  to  the 
slip  rings  ab,  the  machine  will  run  as  a  synchronous  motor 
while  at  the  same  time  direct  current  may  be  obtained  from  the 
brushes  at  the  commutator  end.     The  resultant  current  in  the 
armature  will  be  the  difference  between  the  direct  current   Id 
drawn  from  the  machine  and  the  alternating  current  Ia  which  the 
machine  takes  from  the  power  mains. 

There  is  a  fixed  ratio  between  the  voltages  of  the  direct  and 
alternating  current  sides  of  the  machine.  The  voltage  between 
the  slip  rings  a  and  b  is  a  maximum  when  x  and  y  are  in  the 
neutral  position  and  this  must  be  the  voltage  between  the 
brushes  c  and  d  of  the  direct -current  side  therefore  Ed,  the 
voltage  of  the  direct-current  end,  is  equal  to  the  maximum  volt- 


ART.  351] 


MOTOR-GENERATOR  SETS 


319 


age  at  the  alternating-current  end  or  to  ^2Ea  where  Ea  is  the 
effective  voltage  at  the  alternating-current  end. 

It  is  impossible  to  change  the  voltage  of  the  direct-current  side 
of  the  machine  without  changing  the  alternating  applied  voltage. 
A  change  in  the  field  excitation  for  example  does  not  change  the 
applied  voltage,  but  it  does  change  the  phase  angle  of  the  current 
the  converter  takes  from  the  line,  see  page  256,  and  a  converter 
can  be  used  for  power  factor  correction  in  the  same  way  as  a 
synchronous  motor,  see  page  257. 

If  it  is  desired  to  raise  the  voltage  of  the  direct-current  side  of 
the  machine  then  the  alternating  applied  voltage  must  be  raised. 


Synchronous  Motor  Direct  Current  Generator 

FIG.  381. 


Rotary  Converter 

FIG.  382. — Diagrammatic  representation  of  a  rotary  converter. 

One  method  of  doing  this  is  to  supply  the  rotary  converter  through 
a  transformer  which  has  taps  on  the  secondary  side  by  means  of 
which  the  voltage  may  be  raised  or  lowered.  Another  method  is 
to  insert  a  booster,  generally  on  the  alternating-current  side  of 
the  machine.  This-booster  consists  of  a  small  alternator,  on  the 
same  shaft  as  the  rotary  converter,  the  voltage  of  which  may  be 
added  to  or  subtracted  from  that  of  the  alternating-current  power 
mains. 

351.   Motor-Generator    Sets   and   Rotary   Converters. —  The 
principal  points  of  difference  between  the  rotary  converter,  the 


320    PRINCIPLES  OF  ELECTRICAL  ENGINEERING    [CHAP.XXXIX 

induction  mo  tor -generator  set  and  the  synchronous  motor-gen- 
erator set  are  as  follows: 

Starting. — On  the  alternating- current  side,  the  rotary  con- 
verter operates  as  a  synchronous  motor  and  must  therefore 
be  brought  up  to  speed  and  synchronized,  see  page  258.  The 
induction  motor-generator  set  is  self-starting. 

Parallel  Operation. — A  synchronous  motor  and  a  rotary  con- 
verter are  liable  to  hunt,  see  page  258,  although  this  trouble  is 
practically  eliminated  by  the  use  of  dampers.  Induction  motors 
do  not  hunt. 

Voltage  Variation  on  the  Direct-current  Side. — In  the  case  of 
the  rotary,  a  booster  or  a  boosting  transformer  is  required  to  raise 
the  voltage  of  the  direct-current  side  whereas  the  voltage  of  the 
direct-current  side  of  either  type  of  motor-generator  set  can  be 
controlled  by  the  field  excitation. 

Efficiency  and  Cost. — Being  a  single  machine  instead  of  two 
separate  machines,  the  rotary  converter  is  cheaper  than  either 
type  of  motor-generator  set  and  its  efficiency  is  the  highest  sin.ce 
it  has  only  the  constant  losses  (friction  and  iron  loss)  of  one 
machine. 

Power  Factor  Control. — This  is  possible  with  the  rotary  con- 
verter and  with  the  synchronous  set  but  not  with  the  induction 
motor-generator  set. 

For  small  sizes,  up  to  100  kw.,  the  induction  motor-generator 
set  is  generally  preferred  because  it  is  easily  started  and  because 
the  voltage  of  the  direct-current  end  can  easily  be  regulated. 
For  large  sizes,  500  kw.  and  over,  a  synchronous  machine  is 
preferred  because  it  can  be  used  to  control  the  power  factor  of  the 
system;  if  the  voltage  regulation  is  not  of  great  importance,  as  in 
railway  work,  the  rotary  converter  is  used,  but  if  a  wide  variation 
of  voltage  is  required  from  the  direct-current  side,  a  motor- 
generator  set  is  generally  preferred. 

352.  Polyphase  Rotary  Converter. — The  only  difference  be- 
tween the  single-phase  and  the  polyphase  machine  is  that  the 
former  is  tapped  at  two  points,  as  shown  in  Fig.  382,  whereas  the 
latter  has  to  be  tapped  as  if  the  machine  was  a  polyphase  alter- 
nator of  the  revolving  armature  type,  see  page  241.     A  three- 
phase  rotary  is  shown  in  Fig.  383;  the  armature  is  tapped  at  three 
points,  120  electrical  degrees  apart. 

353.  Split-pole  Rotary  Converter. — One  method  of  changing 
the  voltage  of  the  direct-current  side  of  a  three-phase  rotary  con- 


ART.  354] 


MOTOR-GENERATOR  SETS 


321 


verier  would  be  to  change  the  total  flux  which  passes  between 
ab,  Fig.  384,  without  changing  the  flux  which  passes  between  ac. 
This  is  accomplished  by  splitting  each  pole  as  shown  diagram- 
matically  in  Fig.  384  and  exciting  each  part  separately.  If  then 
the  excitation  of  the  main  pole  M  is  unchanged,  the  flux  threading 
ac  will  be  unchanged,  but  the  total  flux  threading  ab  may  be 
increased  or  decreased  by  varying  the  excitation  of  R,  and  by 
this  means  the  voltage  of  the  direct-current  side  of  the  machine 
may  be  raised  or  lowered  without  any  change  being  required  in 
the  alternating  applied  voltage. 


FIG.  383. — Three-phase  rotary  con-     FIG.  384. — Split-pole     rotary     con- 
verter, verter. 

Another  method  of  changing  from  alternating  to  direct  current 
is  described  on  page  357,  the  piece  of  apparatus  by  which  the 
transformation  is  made  is  called  the  mercury  vapor  converter. 

354.  Frequency  Changers. — To  change  from  one  frequency 
to  another,  a  motor-generator  set  must  be  used  which  consists  of  a 
synchronous  motor  direct  connected  to  an  alternating-current 
generator.  If  60  cycles  have  to  be  obtained  from  a  25-cycle  line, 
then  the  number  of  poles  must  be  in  the  ratio  of  60  to  25,  and 
the  smallest  possible  number  of  poles  would  be  10  for  the  motor 
and  24  for  the  alternator;  this  set  would  run  at  300  r.p.m.,  so 
that  such  frequency  changers  cannot  readily  be  made  for  small 
outputs. 


21 


CHAPTER  XL 
ELECTRIC  TRACTION 

An  electric  car  with  a  seating  capacity  for  fifty  persons  is  gen- 
erally equipped  with  four  motors  each  of  50  h.p.  Such  a  powerful 
equipment  is  required  to  obtain  the  high  speeds  and  high  rates 
of  acceleration  used  in  electric  traction. 

355.  Tractive  Effort. — The  cycle  of  operations  of  an  electric 
car  with  a  reasonable  distance  between  stops  is  shown  in  Fig.  385. 
During  the  time  interval  oa,  energy  is  put  into  the  motors  and  the 
car  is  accelerating;  during  the  interval  ab,  the  motor  circuit  is 


6  c    \stop 

FIG.  385. — Cycle  of  operations  of  an  electric  car. 

open  and  the  car  is  coasting;  during  the  interval  be  the  brakes 
are  applied  and  the  car  gradually  brought  to  rest. 

If  W  is  the  weight  of  the  train  or  car  in  tons  (2000  Ib.)  and  a 
is  the  acceleration  in  miles  per  hour  per  sec.  then  the  tractive 
effort  in  Ib.,  required  for  acceleration 

Weight  in  Ib.  .  , .       .     f, 

=  X  acceleration  in  ft.  per  sec.  per  sec. 

g 

(W  X  2000)       aX^  5280 

32.2  3600 

=  90  W  X  a 

An  additional  accelerating  tractive  effort  of  about  10  per  cent,  is 
required  to  supply  the  rotational  kinetic  energy  of  the  gears, 
motor  armatures  and  other  rotating  parts  so  that  the  tractive 
effort  required  for  acceleration  =  100  W  X  a  =  100  Ib.  per  ton 
for  each  mile  per  hour  per  sec.  of  acceleration;  an  acceleration 
of  1.5  miles  per  hour  per  sec.  may  be  used  without  discomfort 
to  the  passengers. 

The  tractive  effort  to  overcome  train  friction  (bearing,  air  and 

322 


ART.  356]  ELECTRIC  TRACTION  323 

rolling  friction)  is  expressed  in  pounds  per  ton  weight  of  train 
and  is  given  by  the  empirical  formula 

50  72  /        n  __  i\i 

f  =    ^  +  0.03  V  +  0.002  A  YW  (l  +    1Q 


where  /  is  the  tractive  effort  to  overcome  train  friction  in  Ib.  per 

ton  weight  of  train 

W  is  the  weight  of  the  train  in  tons  (2000  Ib.) 
V  is  the  velocity  of  the  train  in  miles  per  hour 
A  is  the  cross  section  of  the  car  above  the  axle  in  sq.  ft. 

and  is  generally  about  120 

n  is  the  number  of  cars;  the  side  friction  of  each  additional 
car  increases  the  air  friction  by  10  per  cent. 

When  there  is  a  curve  in  the  track,  an  additional  tractive  effort 
of  0.7  Ib.  per  ton  for  each  degree  of  curvature  is  required  due  to  the 
increased  rolling  friction;  the  number  of  degrees  of  curvature  is 
equal  to  5730  divided  by  the  radius  of  the  curve  in  feet.. 

When  there  is  a  grade  an  additional  tractive  effort  of  20  Ib. 
per  ton  is  required  for  each  1  per  cent,  of  grade.  When  the 
grade  is  1  per  cent,  the  car  has  to  be  lifted  through  1/100  of  the 
distance  it  travels  and  this  is  equivalent  to  lifting  1/100  of  the 
weight,  or  20  Ib.  per  ton,  through  the  whole  distance  travelled. 

The  maximum  tractive  effort  that  can  be  used  without  causing 
the  driving  wheels  to  slip  is  about  22  per  cent,  of  the  weight  on 
these  wheels,  since  maximum  tractive  effort  =  M  X  weight  on  the 
driving  wheels,  where  M,  called  the  coefficient  of  adhesion,  has  the 
following  values: 

for  a  clean  dry  rail  =0.28 

a  greasy  moist  rail  =  O.lo  or  when  rail  sanded  =  0.25 

a  dry  snow  covered  rail  =  0.11  or  when  rail  sanded  =  0.15 

356.  Speed  Time  Curve. — The  characteristics  of  a  traction 
oad  can  best  be  shown  by  the  working  out  of  an  actual  example. 

A  30-ton  suburban  car  is  equipped  with  four  50-h.p.  motors  having  the 
characteristics  shown  in  Fig.  386.  The  acceleration  has  to  be  1.25  and  the 
deceleration  1.5  miles  per  hour  per  sec.;  the  road  is  level  and  without  curves 
and  the  distance  between  stops  0.75  miles,  the  time  of  the  stop  9  sec.  and  the 

1  Electric  Traction  by  A.  H.  Armstrong,  Standard  Handbook  for  Electrical 
Engineers. 


324       PRINCIPLES  OF  ELECTRICAL  ENGINEERING       [CHAP. 


schedule  speed  21  miles  per  hour.     It  is  required  to  draw  the  speed  time 
curve. 

50 


°-03T/  +  ao02  x 


x 


=  9  +  0.3  +  0.8  =  10.1  Ib.  per  ton  at  10  miles  per  hour 
=  9  +  0.6  +  3.2  =  12.8  Ib.  per  ton  at  20  miles  per  hour 
=  9  +  0.9  +  7.2  =  17.1  Ib.  per  ton  at  30  miles  per  hour 

Now  the  effective  tractive  effort  for  an  accleration  of 
1.25  miles  per  hr.  per  sec. 
=  125  Ib.  per  ton. 

The  train  friction  during  acceleration  will  be  approximately 
=  11  Ib.  per  ton. 

i  herefore  the  total  tractive  effort  required 

=  136  Ib.  per  ton  =  136  X    /t    =1020  Ib.  per  motor. 

Corresponding  to  this  value  on  Fig.  386, 
the  speed  with  550  volts  =  19  miles  per  hour 
and  the  current  per  motor  =  100  amp. 


35        1400 


r 
8 

ds 

K 
g 


ort  in  P 

SS  o 
I  g 


Speed  in  M 
o.  S  & 
Tractive  E 

to  •*>• 
g  g 


100     120 


Amp  ere  s 


FIG.  386. — Characteristics  of  a  50-horse-power,  550-volt  direct-current 
motor,  the  tractive  effort  and  the  speed  being  measured  at  the  rim  of  the 
driving  wheels  of  the  car. 

The  motor  is  so  controlled  that  the  current  remains  constant  at  100  amp. 
per  motor  and  the  acceleration  has  a  constant  value  of  1.25  miles  per 
hour  per  sec.  until  the  speed  has  become  19  miles  per  hour;  the  starting 
resistance  has  then  all  been  cut  out  and  normal  voltage  is  applied  to  the 
motors. 

The  time  taken  to  attain  this  speed  =  vel./accel.  =  19/1.25  =  15. 2  sec. 

.    ,     .        ,  .    x.  19  X5280  .     15.2 

and  the  distance  covered  during  this  time  =  ~ 

from  these  figures  the  points  a,  ai  and  a2  are  plotted  in  Fig.  387. 

The  motor,  running  on  normal  voltage,  will  now  speed  up,  and  the  current 
will  decrease.     Let  the  current  decrease  to  60  amp.  then: 
The  corresponding  tractive  effort  =  500  Ib.  per  motor  and  the  speed  =  23 
miles  per  hour,  Fig.  386. 


X  3600  -  211ft. 


ART.  356] 


ELECTRIC  TRACTION 


325 


4. 6  sec. 


The  average  tractive  effort  while  the  current  is  decreasing   =          ~~o"~ 

=  760  Ib.  per  motor  =  760  X  4/30  =  101  Ib.  per  ton 
The  train  friction  at  21  miles  per  hour  =  13  Ib.  per  ton 
The  tractive  effort  available  for  acceleration  =  88  Ib.  per  ton 

The  average  acceleration  =  88/100  =  0.88  mile  per  hour  per  sec. 

23 19 

The  time  taken  for  the  speed  to  attain  23  miles  per  hour  =     A  o  o 

23  -1-19        5280 
The  distance  covered  during  this  time  =    — ~ —     X  van?)  X  4.( 

The  distance  covered  from  the  start  =211  +  142  =  353  ft. 

From  these  figures  the  points  6,  bi,  and  62,  are  plotted  in  Fig.  387;  points 
c  for  a  current  of  40  amp.  and  d  for  30  amp.  are  determined  in  a  similar 
way. 

Now  the  schedule  speed  is  21  miles  per  hour. 

The  distance  covered  is  0.75  mile. 


36 

32 

,-.-- 

—— 

--rr 
\ 

d, 

4800 

"^ 

-"" 

d> 

^•~~ 

4200 

b  94 

X 

-  f 

*d, 

^ 

^ 

\ 

/ 

*^" 

•^ 

3600 

100    el  90 

a  i/ 

£ 

/£ 

< 

\ 
\ 

^\ 

£ 
3000  a 

l\ 

<?' 

^ 

\ 

2400  § 

|   fi0   «    19 

l\ 

^ 

^ 

\ 

GO 

S 

1800 

/ 

6 

^ 

rent 
C 

/ 

\ 

1200 

^ 

9— 



\ 

S 

d 

\ 

600 

1^ 

-^ 

b2 

\ 

10      20       30       40       50       60       70       80       90      100     110      120 
Seconds 

FIG.  387. — Speed-time  curve  of  an  electric  car. 

The  time  per  cycle  =  0.75/21  =  0.0356  hour  =  129  sec.  of  which  120 
sec.  is  the  running  time  and  9  sec.  the  time  of  the  stop. 

In  order  to  bring  the  car  to  rest  in  120  sec.  from  the  start  the  brakes 
must  be  applied  at  some  point  /  such  that  the  deceleration  will  be  1.5 
miles  per  hour  per  sec. 

If  the  distance  curve  be  now  completed  it  will  be  found  that  the  car 
has  travelled  a  distance  of  4500  ft.  In  order  that  this  distance  be  0.75 
mile,  power  must  be  taken  off  the  car  at  some  point  g  and  the  car  allowed 
to  coast  thereby  reducing  the  average  speed  of  the  run.  During  coasting, 
the  speed  will  fall  off  according  to  the  curve  gh.  The  slope  of  this  line  is 
determined  as  follows:  the  average  velocity  during  coasting  is  25  miles  per 
hour  and  the  corresponding  retarding  force  due  to  train  friction  is  15  Ib. 
per  ton  which  will  cause  a  deceleration  of  0.15  mile  per  hour  per  sec.  or  a 
decrease  in  speed  of  1.5  miles  every  10  sec. 


326       PRINCIPLES  OF  ELECTRICAL  ENGINEERING       [CHAP.  XL 


357.  Energy  Required  by  a  Car.  —  The  series  parallel  method 
of  control,  page  126,  is  used  for  railway  motors  so  that  between 
a  and  6,  Fig.  388,  the  current  taken  from  the  line  is  twice  the  cur- 
rent per  motor  while  between  6  and  c  four  times  the  current  per 
motor  is  taken  from  the  line. 

The  average  current  per  cycle  =  225  amp. 

225  X  550 


The  average  power  per  cycle 
The  energy  per  cycle 


1000 


=  123  kw, 


123  000  X  44 
' 


watt-hr. 


It  is  convenient  to  express  the  energy  required  by  a  car  or 


Amperes  from  Line  for  Four  Motors 

1  1  1  i 

I 

1 

- 

1 

—  • 

—^ 

^~ 

.  — 

—  • 

X 

v. 

6 

c 

10       20        30       40        50      6C 
Seconds 

0         2         46         8         10 

Distance  between  Stops  in  Miles 


FIG.  388. — Current  taken  by  an  electric    FIG.    389. — Energy  consumed 
car  during  one  cycle.  by  an  electric  car. 

train  in  watt-hours  per  ton  mile  or  in  kilowatt-hours  per  car  mile. 
These  quantities  are  determined  in  the  following  way: 

The  energy  per  cycle  =  1500  watt-hours 

The  distance  travelled  per  cycle  =  0.75  mile 

The  weight  of  the  car  =  30  tons 

The  watt-hours  per  ton  mile    =  TT^FCTOQ  =  67 

The  kilowatt-hours  per  car  mile  =  y     ^X  ^^  =  2.0. 

u. /o 

These  two  quantities  decrease  as  the  distance  between  stops 
increases,  as  shown  in  Fig.  389,  because  then  the  energy  required 
to  accelerate  the  car  becomes  a  smaller  portion  of  the  total  energy 
per  cycle.  In  the  extreme  case  of  city  service,  the  coasting  period 
is  generally  eliminated,  the  distance  between  stops  being  so  short 
that  only  acceleration  and  braking  are  required. 

From  curves  such  as  those  in  Fig.  389,  the  power  house  capacity 


AKT.  358] 


ELECTRIC  TRACTION 


327 


may  be  determined  once  the  number  of  cars  and  their  schedule 
has  been  fixed. 

358.  Characteristics  Desired  in  Railway  Motors. — The  series 
motor  is  particularly  suited  for  traction  service,  see  page  103, 
because  it  develops  the  large  starting  torque  required  with  the 
minimum  current,  see  page  102.  In  addition,  it  takes  light  loads 
at  a  high  speed  and  heavy  loads  at  a  slow  speed,  slows  down  on  an 
up  grade  and  speeds  up  on  a  down  grade,  it  therefore  maintains 
the  load  on  the  power  house  more  uniform  than  if  constant  speed 
motors  such  as  the  direct- current  shunt  motor  or  the  alternating- 
current  induction  motor  were  used. 

The  alternating-current  series  motor,  see  page  310,  has  the  same 
characteristics  as  the  direct- current  series  motor  but  is  more 


Mcrin 
Poles 


FIG.  390. — Direct-current  street  railway  motor. 

expensive,  has  a  lower  efficiency  and  does  not  commutate  so  well. 
The  method  of  control  is  simpler- however,  the  low  voltage  re- 
quired for  starting  being  obtained  by  means  of  an  autotrans- 
former  instead  of  by  the  use  of  resistance  and  the  series-parallel 
connection.  These  motors  can  run  on  direct  as  well  as  on  alter- 
nating current  circuits. 

For  one  particular  kind  of  service  the  polyphase  induction 
motor  is  specially  suited,  even  although  it  is  a  constant  speed 
motor,  and  that  is  for  mountain  grade  work  where  the  grades  are 
long  and  fairly  uniform.  When  the  car  is  on  the  down  grade,  the 
motors  tend  to  speed  up  and  run  above  synchronous  speed,  and, 
under  these  conditions  they  become  induction  generators  and 
supply  power  back, to  the  line.  With  a  speed  of  4  per  cent,  above 
synchronous  speed,  the  machine  will  deliver  its  full-load  rating 


328       PRINCIPLES  OF  ELECTRICAL  ENGINEERING       [CHAP.  XL 


ART.  361]  ELECTRIC  TRACTION  329 

to  the  line,  so  that  braking  with  brake  shoes  is  not  necessary  and 
the  danger  of  trouble  on  a  long  grade  due  to  overheating  of  brake 
shoes  is  practically  eliminated. 

359.  Motor  Construction. — To  keep  down  the  size  of  the  motors 
they  must  run  at  a  high  speed,  see  page  101,  so  that  gears  are 
generally  necessary.     The  standard  construction  for  urban  and 
interurban  cars  is  shown  in  Fig.  390. 

Motors  which  are  placed  between  the  driving  wheels,  as  are  all 
street  car  motors,  are  restricted  in  size.  This  restriction  is  most 
keenly  felt  in  the  design  of  large  horse-power  slow-speed  motors 
for  electric  locomotives  for  freight  service.  One  method  of 
providing  increased  space  for  the  motors  is  shown  in  Fig.  391 
where  two  motors  each  of  2000  h.p.  are  mounted  in  the  car 
and  are  connected  to  the  driving  wheels  through  side  rods. 

360.  Distribution  to  the  Cars. — The  three  standard  railway 
systems  are  shown  diagrammatically  in  Figs.  392,  393  and  394. 1 

With  the  single-phase  system  the  trolley  voltage  may  be  as 
high  as  11,000  volts  and  in  order  to  render  high  speed  collection 
reliable  with  long  spans  the  catenary  construction  is  used,  the 
trolley  wire  being  carried  from  a  steel  messenger  cable,  the  latter 
having  a  sag  while  the  former  lies  parallel  with  the  rails. 

Because  of  commutation  troubles  the  standard  direct-current 
voltage  is  not  higher  than  600  volts  although  there  are  lines  in 
operation  at  1200  volts  and  a  few  experimental  lines  at  2400  volts. 
Large  currents  are  therefore  necessary  for  electric  trains  and  to 
carry  this  current  a  third  rail  insulated  from  the  ground  is  used 
instead  of  the  trolley  wire  if  the  trains  run  on  a  private  right  of 
way.  For  city  service  the  familiar  trolley  is  used  and  the 
trolley  system  is  supplied  by  feeders  from  substations. 

The  return  circuit  is  through  the  rails  in  each  case.  This  cir- 
cuit is  made  as  highly  conducting  as  possible  by  bridging  all  the 
rail  joints  with  flexible  pieces  of  copper  called  bonds  which  are 
electrically  welded  to  the  side  of  the  rail. 

361.  Alternating-  and  Direct-current  Traction. — The  single- 
phase  alternating  current  system  has  the  advantage  that  voltages 
as  high  as  11,000  volts  may  be  used  on  the  trolley  so  that  the 
current  in  the  trolley  wire  is  small  and  power  may  be  transmitted 
for  long  distances  before  the  voltage  drop  becomes  too  large, 
the  substations  therefore  are  few  in  number  and  contain  merely 

1  Taken  from  a  paper  by  George  Westinghouse,  in  the  Trans,  of  Ameri- 
can Soc.  of  Mech.  Eng.,  July  1910. 
22 


330       PRINCIPLES  OF  ELECTRICAL  ENGINEERING       [CHAP.  XL 


o 

OD 

.3 

P- 

^i 

Ti 


ART.  361] 


ELECTRIC  TRACTION 


331 


332       PRINCIPLES  OF  ELECTRICAL  ENGINEERING       [CHAP.  XL 

the  necessary  step-down  transformers.  For  the  direct-current 
system,  on  the  other  hand,  at  600  volts,  substations  must  be  closer 
together  to  maintain  the  trolley  voltage  and  these  substations 
contain  rotary  converters  as  well  as  step-down  transformers. 

The  principal  disadvantages  of  the  alternating-current  system 
are  that  the  motors  are  heavier  than  direct-current  motors  of 
the  same  horse-power,  while  the  alternating  magnetic  flux  link- 
ing the  trolley  causes  interference  with  adjoining  telephone 
systems. 

362.  Motor  Car  Trains. — A  number  of  cars  each  with  its  own 
motors  and  controller  may  be  joined  together  to  form  a  train 
which,  by  means  of  the  multiple  unit  system  of  control,  see  page 
133,  can  be  controlled  by  a  single  operator  at  the  head  of  the  train. 
This  system  of  operation  is  very  flexible  because  cars  can  be 
added  to  suit  the  traffic  while  trains  can  readily  be  split  up  at 
junctions  or  single  cars  can  be  attached  or  dropped  as  required 
without  the  necessity  of  stopping  the  whole  train. 

The  acceleration  can  be  rapid  because  all  the  weight  is  on 
the  drivers  and  a  large  draw  bar  pull  can  be  obtained.  Heavier 
trains  can  also  be  run  at  higher  speeds  than  by  locomotives,  while 
the  wear  and  tear  on  the  track  is  reduced  because  the  weight  is  so 
well  distributed.  Such  trains  can  be  run  in  either  direction 
without  having  to  be  turned  end  for  end. 

363.  Electric  Locomotives. — When  the  cars  of  a  steam  road 
have  to  be  hauled  into  the  city,  through  tunnels,  or  on  mountain 
grades,  electric  locomotives  are  often  used.     Their  capacity  is  not 
limited  by  grate  area  and  boiler  capacity  so  that  they  can  develop 
very  large  torques  for  starting  and  accelerating  and  can  generally 
give  50  per  cent,  more  draw  bar  pull  than  a  steam  locomotive 
of  the  same  weight.     The  torque  also  is  uniform,  such  locomo- 
tives can  therefore  give  high  acceleration  and  high  schedule 
speeds. 

ELECTRIC   HOISTING 

364.  Crane  and  Hoist  Motors. — In  the  case  of  cranes,  where 
the  load  is  visible  at  all  times  and  where  the  starting  and  accelerat- 
ing of  the  load  is  a  large  part  of  the  total  hoisting  cycle,  the 
direct  current  series  motor  is  used  because  of  its  good  starting 
characteristics.     Where   only   alternating   current   is   available, 
the  wound  rotor  polyphase  induction  motor  is  generally  used,  but 
it  is  not  such  a  satisfactory  machine,  see  page  298. 


AHT.  365]  ELECTRIC  TRACTION  333 

Motors  for  large  cranes,  such  as  those  used  in  rolling  mills, 
take  large  currents  and  are  generally  controlled  by  magnetic 
switch  controllers. 

When  the  load  has  to  be  raised  a  considerable  distance,  as  in 
the  case  of  mine  hoisting,  the  load  is  accelerated  for  only  a  small 
portion  of  the  total  cycle  and  is  run  thereafter  at  a  constant  speed 
not  greater  than  that  fixed  by  law.  For  such  service  the  direct- 
current  compound  motor  is  suitable  because  its  speed  cannot 
exceed  a  safe  maximum  value.  Where  only  alternating  current 
is  available,  the  polyphase  wound  rotor  induction  motor  is  used. 

365.  Braking. — The  brakes  used  on  cranes  and  hoisting 
machines  must  be  so  constructed  that  they  will  set  if  there  is  any 
interruption  in  the  current  supply.  Such  a  brake  is  shown  in 
Fig.  395.  When  power  is  switched  on  to  start  the  hoisting  motor, 
the  solenoid  lifts  the  lever  L 
and  releases  the  brake,  but  as 
soon  as  current  ceases  to  flow, 
the  weight  W  sets  the  brake. 

When  a  light  load  is  being 
lowered,    power  must  be  ap- 
plied to  drive  it  down.      When     FlG  395._Brake  for  hoists  and  cranes, 
the  load  is  heavy,  it  may  be 

allowed  to  overhaul  the  motor,  the  speed  being  kept  within  rea- 
sonable limits  by  means  of  the  brake.  This  causes  jerky  opera- 
tion, and  excessive  wear  of  the  brake,  and,  for  coal  and  ore  un- 
loaders  and  other  such  hoists  doing  fast  hoisting  service,  mechan- 
ical braking  is  not  very  satisfactory  and  dynamic  braking  is  now 
used. 

The  hoisting  motors  for  such  hoists  are  direct-current  compound 
wound  and  are  connected  as  in  Fig.  396  during  the  hoisting  period. 
When  being  lowered,  the  empty  bucket  is  allowed  to  overhaul  the 
motor,  which  is  connected  as  shown  in  Fig.  397,  the  shunt  coils 
being  separately  excited  while  the  armature  is  short  circuited 
through  an  adjustable  resistance  R.  The  machine  then  acts  as  a 
generator  driven  by  the  descending  load  and  sends  a  current  7 
through  the  resistance  R.  As  the  machine  speeds  up,  the  current 
/  increases  until  the  retarding  torque  due  to  the  current  I  becomes 
equal  to  the  effective  torque  driving  the  armature. 

To  raise  the  lowering  speed,  the  resistance  R  must  be  increased. 
This  reduces  the  current  7  and  the  braking  torque  temporarily, 
and  the  speed  increases  until  the  greater  e.m.f.  generated  in  the 


334      PRINCIPLES  OF  ELECTRICAL  ENGINEERING        [CHAP.  XL 

armature  is  again  able  to  send  the  necessary  current  I  through 
the  higher  resistance.  By  varying  the  resistance  R,  the  lowering 
speed  may  be  varied  from  almost  zero  to  any  desired  value.  To 
stop  the  load,  a  mechanical  brake  must'be  applied,  since  dynamic 
braking  can  only  take  place  while  the  armature  is  moving. 


FIG.  396. — Hoisting  period,  FIG.  397. — Lowering    period,     with 

dynamic  braking. 

FIGS.  396  and  397. — Connections    of    a    compound-wound    direct-current 

hoist  motor. 

Dynamic  braking  is  also  applied  to  crane  motors.  During 
hoisting,  the  motor  is  connected  as  shown  in  Fig.  398  On  the  first 
step  of  the  controller  about  two-thirds  of  full-load  current  flows 
through  the  circuit,  and  this  is  sufficient  to  release  the  brake  and 
start  the  average  load. 


FIG.  398.— Hoisting.     FIG.  399.— Lowering.     FIG.  400.— Dynamic 

braking. 
FIGS.  398  to  400. — Connection  of  a  series-wound  direct-current  crane  motor. 

When  lowering,  the  machine  is  changed  over  to  a  shunt  machine 
as  shown  in  Fig.  399  and  a  resistance  R  is  inserted  to  limit  the 
flow  of  current  in  the  field  coil  circuit  to  two-thirds  of  full-load 
current,  which  is  sufficient  to  release  the  brake.  The  direction  of 


ART.  366] 


ELECTRIC  TRACTION 


335 


the  field  current  is  the  same  as  during  hoisting,  but  the  armature 
terminals  are  reversed  so  that  the  armature  current  is  reversed 
and  the  motor  drives  the  load  down.  If  the  load  is  heavy  enough 
to  overhaul  the  motor,  then  the  motor  speeds  up  and  its  back 
e.m.f.  increases,  until  finally  the  machine  runs  as  a  generator  and 
acts  as  a  brake.  The  current  Ia  is  now  reversed  and  the  machine 
supplies  its  own  exciting  current,  so  that  it  may  be  disconnected 
from  the  line  as  shown  in  Fig.  400  and  the  braking  speed  adjusted 
by  means  of  the  rheostat  Rb- 

366.  Flywheel  Motor  Generator  Sets  for  Mine  Hoisting.— The 
load  on  the  motor  of  a  hoist  is  extremely  variable,  as  shown  by 
curve  A,  Fig.  401,  and,  when  the  motor  has  an  output  of  500 
h.p.  or  more  as  in  the  case  of  motors  for  mine  hoists,  such 
a  load  is  not  a  very  desirable  one  for  a  power  company,  and  a 


I§  *I1  111  |  11  §1§  |  |  III  §§g  2S 

Time  in  Seconds 

Curve  A    Current  taken  by  hoist  motor. 
Curve  B    Current  taken  by  induction  motor. 

FIG.  401. — Characteristics   of   a  hoisting  load   when   a   fly-wheel  motor- 
generator  set  is  used. 

cheaper  rate  for  power  can  generally  be  obtained  if  the  load  is 
more  uniform. 

To  obtain  this  result  and  at  the  same  time  to  reduce  the  large 
losses  in  the  starting  rheostat,  the  Ward  Leonard  system,  see 
page  110,  is  used.  The  high  speed  motor  generator  set  consists 
of  a  wound  rotor  induction  motor  which  takes  power  from  the 
transmission  line,  and  a  direct-current  generator  which  supplies 
power  to  the  hoisting  motor,  while  a  heavy  flywheel  is  mounted 
on  the  same  shaft  as  shown  in  Figs.  402  and  403. 

The  excitation  of  the  hoisting  motor  is  kept  constant  by  means 
of  the  exciter  E,  while  the  speed  of  the  motor  is  controlled  by  the 
resistance  r  in  the  generator  field  circuit,  by  means  of  which  the 
voltage  Et  applied  to  the  motor  terminals  may  be  varied. 

As  the  load  on  the  hoist  motor  increases,  the  current  in  the 
induction  motor  leads  tends  to  increase  in  the  same  ratio,  but  a 
slight  increase  in  the  current  /  causes  the  plunger  p  of  the  sole- 


336      PRINCIPLES  OF  ELECTRICAL  ENGINEERING      [CHAP.  XLI 


noid  $  to  raise  the  plates  of  the  water  rheostat  W  thereby  increas- 
ing the  resistance  in  the  rotor  circuit,  so  that  the  rotor  current 
and  torque  are  maintained  constant.  This  torque  is  not  sufficient 


Induction  Motor       D.C. Generator 


Exciter 


FIG.  402. — Flywheel  motor-generator  set  for  the  operation  of  mine  hoists. 

for  the  load,  so  that  the  speed  drops  and  thereby  causes  the  fly  - 
wheel  to  give  up  energy. 


Flywheel  Motor-Ge»erator  Set 

FIG.  403. — Connections  for  a  hoist  motor  supplied  by  a  fly-wheel  motor 

generator  set. 

If  the  load  on  the  hoist  motor  now  decreases,  the  current  I 
tends  to  decrease,  but  a  slight  decrease  in  this  current  causes  the 
pull  of  solenoid  S  to  decrease  and  the  plates  of  the  water  rheostat 


ART.  367]  ELECTRIC  TRACTION  337 

to  drop,  thereby  decreasing  the  resistance  in  the  rotor  circuit,  so 
that  the  rotor  current  and  the  rotor  torque  are  maintained. 
This  torque  is  greater  than  necessary  for  the  load  and  so  acceler- 
ates the  flywheel. 

By  means  of  such  a  slip  regulator,  the  power  taken  from  the 
line  is  maintained  approximately  constant,  as  shown  in  curve  B, 
Fig.  401. 

367.  Safety  Devices. — The  safety  brake  is  of  the  type  shown 
in  Fig.  395  and  will  set  and  hold  the  load  as  soon  as  current  ceases 
to  flow  in  the  solenoid  B. 

If  for  example  the  field  circuit  of  the  hoisting  motor  were  to 
open,  the  current  in  the  solenoid  B  would  be  interrupted  and  the 
brake  would  set. 

If  the  cage  overtravels,  it  closes  the  switch  T  in  the  hatchway 
and  this  shunts  the  current  from  the  solenoid  B  and  allows  the 
brake  to  set.  The  resulting  overload  then  opens  the  circuit 
breaker  C. 

The  solenoid  B  is  also  shunted  when,  due  to  an  excessive  over- 
load, the  circuit  breaker  C  opens  and  closes  the  contacts  ab.  A 
similar  pair  of  contacts  cd  ensure  that  the  brake  shall  be  set  when 
the  main  switch  is  open. 


CHAPTER   XLI 
TRANSMISSION  AND  DISTRIBUTION 

368.  Direct-current  Stations. — When  direct  current  is  used, 
the  voltage  can  be  transformed  up  and  down  only  by  means  of 
motor  generator  sets  and,  since  these  are  expensive  and  require 
supervision,  they  are  seldom  used,  so  that  the  connected  load  of 
motors  and  lamps  must  operate  at  the  power  house  voltage. 

The  110- volt  lamp  is  practically  standard  because  it  has  a 
stronger  filament  than  have  lamps  of  higher  voltage.  The 
use  of  such  a  low  voltage  necessitates  the  use  of  conductors  of 
large  cross  section  to  carry  the  current,  and  higher  voltages  are 
desirable. 

If  50  kw.  has  to  be  transmitted  a  distance  of  100  yd.  with  a  drop  in  vol- 
tage not  exceeding  2.5  per  cent.,  find  the  necessary  cross  section  of  the  con- 
ductor if  the  voltage  is  110  volts  and  also  if  it  is  220  volts.  The  resistance 
of  copper  is  taken  as  11  ohms  per  cir.  mil  foot. 


At  110  volts 

At  220  volts 

50.X  1000       . 

50  X  1000 

Voltage    drop    in    the 
line. 

Resistance  of  the  line.  . 

110          -455  amp. 

=  2.5%  of    110   =  2.75 

volts 
o  yc 
=  4"g5-  =  0.006  ohms 

_  11X200X3  -0006 

220        ~         amp. 
=   2.5%  of    220    =   5.5 
volts 

=  ~  =  0.024  ohms 
11X200X3 

of  wire. 
Cross  section  of  wire  in 
cir.  mils. 

cir.  mils 
=  1,100,000 

cir.  mils 
=  275,000 

The  cross  section  of  the  wire  is  inversely  proportional  to  the 
square  of  the  voltage  for  the  same  loss  in  the  line. 

If  the  three- wire  system  of  distribution  is  used  then  110  volt 
lamps  may  be  operated  from  220  volt  mains  as  shown  in  Fig.  376, 
page  316.  The  neutral  wire  may  be  small  in  cross  section  if  the 
load  is  nearly  balanced  under  all  conditions  of  loading. 

Where  there  is  only  a  small  lighting  load,  or  where  the  lights 

338 


ART.  369]          TRANSMISSION  AND  DISTRIBUTION 


339 


are  supplied  from  special  mains,  then  a  voltage  of  550  may  be 
used  for  the  motors.  Voltages  greater  than  550  are  dangerous; 
even  110  volts  may  prove  fatal  to  a  person  who  happens  to  make 
unusually  good  contact  with  the  mains. 

369.  Alternating-current  Stations. — Where  power  has  to  be 
transmitted  a  long  distance  or  has  to  be  distributed  over  a  wide 
area,  the  alternating-current  system  is  preferred  because  the 
voltage  can  be  transformed  up  and  down  by  means  of  transform- 
ers. These  require  no  supervision  and  may  be  installed  in  the 
open,  on  poles  or  in  manholes. 

A  typical  high-voltage  transmission  system  is  shown  in  Fig. 
404.  The  power  station  contains  the  generators  and  step-up 


50000  Volts 


Generator 
2200  Volt 


Transformers 


Oil  Switch 


Power  House 


Terminal  Station 


FIG.  404. — Diagrammatic  representation  of  a  high  voltage  transmission 

system. 


transformers  while  the  step- down  transformers  are  placed  in  a 
terminal  station.  The  load  is  carried  by  several  units  operating 
in  parallel  so  that  one  machine  may  be  shut  down  for  repair 
without  affecting  the  total  load. 

From  the  terminal  station,  power  is  transmitted  to  a  number  of 
substations  which  are  conveniently  located  with  respect  to  the 
load.  In  the  case  of  a  transformer  substation,  the  equipment 
consists  of  the  necessary  step-down  transformers  to  reduce  the 
voltage  to  2200  volts  at  which  it  is  supplied  to  the  feeders.  Each 
feeder  is  generally  controlled  by  a  feeder  regulator,  see  page  281, 
by  means  of  which  its  voltage  may  be  adjusted.  These  feeders 


340      PRINCIPLES  OF  ELECTRICAL  ENGINEERING      [CHAP,  xn 

supply  transformers  which  are  placed  on  poles,  or  underground 
in  manholes,  and  step  the  voltage  down  to  110  volts  for  the 
consumer. 

When  it  is  necessary  to  transform  to  direct  current  as  for  exam- 
ple to  supply  power  to  a  large  machine  shop,  the  substation  equip- 
ment is  as  shown  in  Fig.  405.  Power  enters  the  station  over  the 
three  bare  wires  a,  6,  and  c  which  are  carried  through  porcelain 


FIG.  405. — Connections  of  an  A.-C.  to  D.-C.  motor-generator  sub-station. 


wall  bushings  and  are  connected  to  the  high-tension  bus  bars  m, 
n  and  p.  Power  to  operate  the  synchronous  motor  is  taken  from 
these  bus  bars  through  a  delta-connected  bank  of  transformers  by 
which  the  voltage  is  reduced  to  2200  volts,  while  half  voltage 
taps  are  also  supplied  so  that  the  self-starting  motor  may  be 
started  at  half  voltage;  the  double- throw  switch  for  this  purpose 
is  not  shown. 


ART.  371]          TRANSMISSION  AND  DISTRIBUTION  341 

370.  The  voltages  used  in  practice  are : 

Direct  Current: 

110  volts  for  lighting,  generally  obtained  from  a  220- volt  three- 
wire  system. 

110,  220  and  550  volts  for  motors. 

600  volts  for  street  railway  systems. 

1200  volts  for  interurban  systems. 

2400  volts  for  trunk-line  electrification. 
Alternating  Current: 

110  volts  single  phase  for  lighting  and  for  small  motors. 

110,  220,  440  and  550  volts  for  polyphase  motors  up  to  50  h.p. 

440,  550  and  2200  volts  for  polyphase  motors  greater  than 
50  h.p. 

13,200  volts  is  the  highest  voltage  generated  by  alternators; 
low -voltage  alternators  with  step-up  transformers  are  more 
reliable. 

100  volts  per  mile  of  line  with  a  maximum  of  110,000  volts  for 
power  transmission. 

The  tendency  is  to  use  a  frequency  of  60  cycles  for  power  and 
lighting  work  as  it  gives  a  better  choice  of  speeds  for  induction 
motors  than  does  25  cycles,  see  page  297.  For  single-phase  rail- 
way work,  however,  25  cycles  are  necessary  because  of  the  diffi- 
culty in  constructing  motors  that  will  commutate  satisfactorily 
at  a  higher  frequency.  In  the  case  of  cement  mills  and  other 
such  places  where  most  of  the  induction  motors  are  slow-speed 
machines,  25  cycles  may  be  used  with  advantage. 

371.  Comparison    between    single-phase    and    three-phase 
transmission. 

10,000  kw.  at  80  per  cent,  power  factor  has  to  be  delivered  at  the  end  of  a 
single-phase  25-mile  line  at  50,000  volts  and  60  cycles.  The  size  of  wire  is 
No.  000  and  the  wires  are  spaced  72  in.  apart.  Find  the  voltage  at  the 
generating  station  and  also  the  power  loss  in  the  line. 

the  resistance  of  this  wire  =  0.326  ohms  per  mile,  page  216. 
the  reactance  at  60  cycles  =  0 . 742  ohms  per  mile,  page  216. 

10,000  X  1000 
the  current  in  the  line  =  ~o^~>^50  000    =          amP- 

the  resistance  drop  in  50  miles  of  wire  =  IR 

=  250  X  0.326  X  50 
=  4075  volts 

the  reactance  drop  in  50  miles  of  wire  =  IX 

=  250  X  0.742  X  50 
=  9250  volts. 

the  voltage  of  the  generating  station     =  E0,  Fig.  406. 


342      PRINCIPLES  OF  ELECTRICAL  ENGINEERING      [CHAP.  XLI 


the  power  loss  in  the  line 


=    V  (40,000  +  4075)2  +(30,000  +  9250) : 

=  59,000  volts 

=  PR 

=  2502  X  0.326  X  50 

=  1020  kw. 

=  10.2  per  cent,  of  the  output 


15,000  k\v.  at  80  per  cent,  power  factor  has  to  be  delivered  at  the  end  of 
a  three-phase  25-mile  line  at  50,000  volts  and  60  cycles.     The  size  of  wire 


FIG.  406. 


Neutral  Wire  Carries  No  Currer 

FIG.  407. 


is  No.  000  and  the  wires  are  spaced  72  in.  apart.     Find  the  voltage  in  the 
generating  station  and  also  the  power  loss  in  the  line. 

the  resistance  of  the  wire  =  0.326  ohms  per  mile. 
the  reactance  at  60  cycles  =  0.742  ohms  per  mile. 
the  current  in  the  line 


08 


217 


The  problem  is  best  solved  by  considering  each  line  separately  as  shown  in 
Fig.  407  then 

Et,  the  line  voltage  to  neutral  =  50,000/1.73  =  29,000  volts 
the  resistance  drop  in  25  miles  of  wire  =  IR 

=  217  X  0.326  X  25 

=  1770  volts 
the  reactance  drop  in  25  miles  of  wire  =  IX 

=  217  X  0.742  X  25 

=  4020  volts 
the  line  voltage  to  neutral  at  the  generating  station 

=  V  (29,000  X  0.8  +  1770)2  +  (29,000 
X  0.6  +  4020)  2 

=  32,950  volts 
the  voltage  between  lines         =  32,950  X  1.73 

=  57,000  volts 
the  power  loss  in  the  line         =3  X  2172  X  0.326  X  25 

=  1150kw. 

=  7.6  per  cent,  of  the  output. 


ART.  372]          TRANSMISSION  AND  DISTRIBUTION 


343 


372.  Lightning  arresters  are  used  to  protect  electrical  equip- 
ment from  lightning  discharges  and  abnormally  high  voltages 
of  all  kinds. 

The  current  due  to  a  lightning  discharge  has  a  high  frequency 
and  will  not  pass  readily  through  a  reac- 
tance, so  that,  if,  as  in  Fig.  408,  a  resis- 
tance path  R  is  provided  to  ground  with 
an  air  gap  g  long  enough  to  prevent  the 
flow  of  current  under  normal  conditions, 
and  a  reactance  or  choke  coil  C  is  placed 
between  the  line  and  the  equipment  to  be 
protected,  then  a  lightning  discharge  will 
be  held  up  by  the  choke  coil  and  will 
jump  across  the  air  gap  to  ground. 

Once  an  arc  is  started  across  the  gap, 
however,  the  "line  current  will  follow 
through  the  path  abed,  and  provision  must  be  made  in  the 
arrester  to  prevent  this  current  from  passing.  The  current 
may  be  limited  by  inserting  a  resistance  R  in  series  with  the  gap, 
while,  if  the  electrodes  of  the  air  gap  are  made  of  non-arcing  metal, 


F 

K 


FIG.   408. — Connection  of 
lightning  arresters. 


Line 


Arrester 


Blow-out 
Coil 


J  Ground 


FIG.  409.— 3000  volt 
multigap  arrester  for  sta- 
tion installation. 


FIG.  410. — Lightning  arrester  for  direct-current 
circuits. 


that  is  metal  such  as  zinc  which  has  a  low  boiling  point,  then, 
on  an  alternating- current  line,  the  arc  will  not  be  maintained  but 
will  stop  as  the  alternating  current  passes  through  zero.  A  single 
gap  of  non-arcing  metal  will  protect  a  300-volt  line,  for  higher 
voltages,  several  gaps  are  placed  in  series  as  in  the  3000-volt 


344      PRINCIPLES  OF  ELECTRICAL  ENGINEERING      [CHAP.  XLI 


arrester  shown  in  Fig.  409  which  is  really  equivalent  to  three 
arresters  one  with  3  gaps  and  a  high  resistance  in  series,  an- 
other with  6  gaps  and  a  lower  resistance  in  series,  and  the  third 
with  15  gaps  in  series. 

It  is  not  so  easy  to  rupture  a  direct  current  and,  even  when  non- 
arcing  electrodes  are  used,  it  is  necessary  also  to  supply  a  blow-out 
coil  connected  as  shown  in  Fig.  410,  which  is  excited  when  a  power 
current  flows  through  the  arrester. 


Wooden 

Cover  • 

fielded 
Tank 


Horn  Gap 

force/am 
Bussing 


Base 


FIG.   411. — Aluminium  arrester. 

For  the  protection  of  long-distance  high-voltage  transmission 
lines  the  aluminium  arrester  is  generally  used.  An  aluminium 
cell  consisting"  of  two  aluminium  plates  on  which  a  film  of  hydrox- 
ide has  been  formed,  when  immersed  in  a  suitable  electrolyte, 
will  allow  only  a  very  small  current  to  flow,  until  the  voltage 
reaches  a  critical  value.  At  a  higher  voltage,  the  current  that 
can  flow  is  very  large  but  the  high  resistance  is  reestablished  as 


ART.  373] 


TRANSMISSION  AND  DISTRIBUTION 


345 


soon  as  the  voltage  is  reduced  below  the  critical  value.  Such 
a  cell  can  therefore  act  as  a  safety  valve  and  aluminium  arresters 
are  made  up  in  the  form  shown  in  Fig.  411,  about  300  volts  per 
pair  of  plates  being  allowed. 

Even  with  300  volts  between  plates,  a  small  current  flows,  to 
prevent  which,  the  arrester  is  connected  to  the  line  through  a 
horn  gap  as  shown  in  Fig.  404.  When  the  arrester  is  disconnected 
from  the  line,  however,  the  hydroxide  film  dissolves,  so  that  the 
arrester  must  be  charged  daily  by  being  connected  to  the  line 
by  the  closing  of  the  horn  gap  for  a  few  seconds. 


FIG.  412. — Hand-operated,  triple-pole  oil  switch. 

373.  Switches. — Oil  switches  such  as  that  shown  in  Fig.  412 
are  used  to  rupture  the  current  in  high-voltage  lines.  These 
switches  are  generally  kept  at  a  distance  from  the  operator  and 
are  opened  and  closed  through  a  system  of  levers,  or  may  be  of  the 
remote  control  type  operated  by  means  of  a  solenoid  or  by  a  small 
motor.  In  all  cases  the  switch  is  closed  against  the  tension  of  a 
spring  and  is  held  closed  by  means  of  a  latch.  This  latch  may 
be  released  by  an  overload  relay,  so  as  to  allow  the  switch  to  open 
when  the  current  becomes  excessive. 

To  localize  trouble,  important  switches  are  generally  mounted 
as  shown  in  Fig.  413  with  each  pole  in  a  separate  brick  or  concrete 
compartment. 

Disconnecting  switches  such  as  that  shown  in  Fig.  405  are  not 

23 


346       PRINCIPLES  OF  ELECTRICAL  ENGINEERING      [CHAP.  XLI 

intended  to  open  while  current  is  flowing  but  are  used  to  discon- 
nect apparatus  once  the  circuit  has  been  opened  by  an  oil  switch. 
Such  disconnecting  switches  are  opened  and  closed  by  a  long  stick 
with  a  hook  attached  to  the  end. 

374.  Overhead  Line  Construction. — For  voltages  up  to  50,000, 
wooden  poles  with  pin  insulators  are  used  to  support  the  line. 
For  higher  voltages,  pin  insulators  become  very  large  and  the 


FIG.  413. — Motor-operated,  triple-pole  oil  switch. 

stresses  on  the  pin  become  excessive,  so  that  the  suspension  type 
of  insulator  has  to  be  used  and  these  are  generally  suspended  from 
steel  towers  as  shown  in  Fig.  415. 

To  protect  the  line  from  lightning,  it  is  usual  to  run  a  steel  wire 
parallel  to  the  power  wires,  and  to  ground  this  wire  at  every  tower. 
Lightning  will  generally  strike  this  ground  wire  and  pass  to  the 


ART.  375]          TRANSMISSION  AND  DISTRIBUTION 


347 


ground  without  doing  injury,  rather  than  strike  the  power  wires 
and  then  pass  to  ground  through  the  insulators. 

375.  Underground  Construction. — To  carry  current  under- 
ground, stranded  copper  cable  is  used.  The  copper  is  insulated 
with  paper  which  is  then  impregnated  with  a  compound  such  as 


Section  A-D 


T 


FIG.  414. — Wooden  pole  with  pin  in- 
sulators,  and   ground   wire   at   top. 


FIG.  415. — Steel  tower  with  suspen- 
sion insulators. 


resin  oil  after  which  the  cable  is  sheathed  with  lead  which  keeps 
out  moisture  and  at  the  same  time  protects  the  cable  against 
mechanical  injury.  The  cable  has  to  be  flexible  enough  to  bend 
round  corners  because  it  has  to  be  drawn  into  tile  ducts  through 
manholes  such  as  that  shown  in  Fig.  416,  which  manholes  are 
restricted  in  size. 


348       PRINCIPLES  OF  ELECTRICAL  ENGINEERING      [CHAP.  XLI 

The  necessary  cross  section  of  copper  is  generally  fixed  by  the 
permissible  voltage  drop  in  the  case  of  low- voltage  cables,  but  is 
always  fixed  by  heating  in  high- voltage  cables.  A  current 
density  of  1000  amp.  per  sq.  in.  of  copper  section  can 
seldom  be  exceeded,  and  this  requires  8.7  volts  per  500  ft., 
which  is  2.5  per  cent,  of  350  volts,  so  that,  if  the  voltage  drop  is 
limited  to  2.5  per  cent,  and  the  transmission  distance  is  1000  ft., 
then,  for  voltages  less  than  350  volts,  the  current  density  must  be 
less  than  1000  amp.  per  sq.  in.  while  for  voltages  greater  than 
350  volts,  the  drop  in  the  cable  will  be  less  than  2.5  per  cent, 


Street  Level 


FIG.  416.— Manhole. 

376.  Switchboards. — For  convenience  of  manipulation,  all  the 
apparatus  for  controlling  the  machines  and  circuits  in  a  power 
station,  as  well  as  all  the  measuring  instruments,  are  assembled 
as  compactly  as  possible  on  a  switchboard. 

The  method  of  designing  a  switchboard  is  first  of  all  to  lay 
out  the  complete  diagram  of  connections,  then  design  the  front 
of  the  board  placing  the  different  pieces  of  apparatus  in  the  most 
convenient  positions,  after  which  the  connections  on  the  back  of 
the  board  can  be  laid  out  and  any  rearrangement  necessary  can 
then  be  made. 

Fig.  417  shows  the  diagram  of  connections  for  a  single  shunt 
generator  which  supplies  four  feeders. 

Fig.  418  shows  the  front  of  the  board. 

Fig.  419  shows  the  connections  as  they  would  appear  if  the 
slate  front  of  the  board  was  removed.  This  diagram  is  lettered 
similarly  to  Fig.  417. 

In  larger  stations  it  is  usual  to  provide  a  separate  panel  on  the 
switchboard  for  each  machine  and  also  for  each  feeder. 

Fig.  420  shows  a  three  panel  switchboard  for  a  three  phase  alter- 
nator, a  three  phase  feeder  and  an  exciter. 


ART.  376]  TRANSMISSION  AND  DISTRIBUTION 


349 


The  exciter  panel  is  equipped  with : 

1  ammeter  AI. 

I  handwheel  for  the  exciter  rheostat  Ri. 

1  switch  Sj  with  a  fuse  which  is  on  the  back  of  the  panel. 

2  switches  Si  and  $2  for  the  station  lighting  and  other  auxiliary 
circuits. 


F,G.  417.         II      


ircuit 
Voltmeter    Br«*fn    Ammeter 


\^/ 

(~)    Field  Rheostat if) 
jjp|        Handle       d=b 


Main  Switch 


FIG.  418.  FIG.  419. 

FIGS.  417  to  419. — Switchboard    for    a    direct-current   shunt   generator, 
and  four  feeder  circuits. 

The  exciter  voltage  is  indicated  by  the  voltmeter  V\  on  the 
swinging  bracket.  As  the  station  grows  in  size,  a  second  exciter 
will  have  to  be  added  to  operate  in  parallel  with  the  first  but,  by 
means  of  a  plug  P,  the  voltmeter  Vi  may  be  made  to  indicate 
the  voltage  of  each  machine. 


350       PRINCIPLES  OF  ELECTRICAL  ENGINEERING      [CHAP. 


The  generator  panel  is  equipped  with : 
1  three  phase  indicating  wattmeter. 
1  ammeter. 
1  voltmeter. 

1  three  phase  watt-hour  meter,     (called  in  practice  a  record 
ing  wattmeter.) 

1  handwheel  for  the  alternator  field  rheostat  R2. 

1  field  switch  >S3. 

1  triple  pole  single  throw  (T.  P.  S.  T.)  oil  switch. 

1  current  transformer  C. 

2  potential  transformers  V. 


3f&eO>l  SmU 
ilxu/t  Overload 

f/fffajf 


Switchboard  Connection  diagram 

FIG.  420. — Switchboard  with  an  exciter  panel,  a  three-phase  alternator  and 
a  three-phase  feeder  panel. 

As  the  station  grows  in  size,  additional  alternators  have  to  be 
added  and  they  must  operate  in  parallel  so  that  a  synchroscope 
must  be  provided.  This  is  placed  on  the  swinging  bracket  and 
the  necessary  connections  are  made  by  plugs  on  the  generator 
panels.  For  a  single  alternator  a  synchroscope  is  not  required. 

The  generator  oil  switch  has  no  overload  release;  protection 
against  overloads  is  provided  for  by  the  use  of  automatic  switches 
on  the  feeder  circuits. 

The  feeder  panel  is  equipped  with : 

3  ammeters. 

1  T.P.S.T.  oil  switch  with  overload  release. 

3  current  transformers. 


ART.  377]          TRANSMISSION  AND  DISTRIBUTION 


351 


377.  Instrument  Transformers. — The  instruments  in  circuits 
with  a  voltage  of  2300  volts  or  greater  are  not  connected  directly 
in  the  circuit  but  are  connected  through  transformers  as  shown 
diagrammatically  in  Fig.  421. 

T  is  a  potential  transformer  and  is  built  exactly  like  a  standard 
lighting  transformer  but  on  a 
smaller  scale.  The  voltmeter  V 
measures  the  secondary  voltage 
but  is  calibrated  in  terms  of  the 
voltage  of  the  primary  side. 

The  series  transformer  has  the 
primary  side  connected  directly  in 
the  line  and  the  secondary  short 
circuited  by  an  ammeter  or  by  the 
current  coils  of  a  wattmeter. 


FIG.  421. — Connections  of  in- 
strument transformers. 


Since  the  secondary  ampere  turns  of  a  transformer  are  always 
equal  in  number  to  the  primary  ampere  turns  therefore  n2I2  = 

ni/ior/o  =  --/!  so  that  the  current  measured  by  the  instruments 

nz 
is  proportional  to  the  current  in  the  line. 


CHAPTER  XLII 

• 

ELECTRIC   LIGHTING 

A  hot  body  gives  off  radiant  energy  in  the  form  of  heat,  light 
and  chemical  energy  and,  to  maintain  the  temperature  of  the 
body,  energy  must  be  given  to  the  body  at  the  same  rate  as  it  is 
dissipated  by  the  body.  An  illuminant  should  have  as  much  of 
the  total  radiation  as  possible  in  the  form  of  light;  power  is 
required  to  maintain  the  other  radiation  but  no  light  is  obtained 
from  it. 

As  the  temperature  of  a  body  is  raised,  the  color  of  its  light 
changes  from  red  to  white,  while  its  light  efficiency  increases 
rapidly,  so  that  high  temperature  is  a  necessary  condition  for  an 
illuminant  of  the  incandescent  type. 

378.  The  carbon  incandescent  lamp  consists  of  a  filament  of 
carbon  enclosed  in  a  glass  globe  from  which  the  air  has  been  ex- 
hausted.   When  the  temperature  of  such  a  filament  reaches  about 
1600°  C.,  the  rate  of  evaporation  of  the  carbon  becomes  excessive 
and  the  life  of  the  filament  becomes  short.     The  life  of  such  a 
lamp  is  the  number  of  hours  the  lamp  will  burn  before  its  candle 
power  drops  to  80  per  cent,  of  the  original  value;  the  decrease  in 
candle  power  is  due  largely  to  evaporation  of  carbon  from  the 
filament,  this    carbon   deposits  on  the  inside  of  the  globe  and 
blackens  it. 

If  the  voltage  applied  to  a  carbon  lamp  increases,  the  current 
increases,  and  so  also  does  the  temperature  of  the  filament  and 
the  efficiency  of  the  lamp,  but  the  life  is  reduced.  A  standard 
16  candle-power,  110  volt  lamp  takes  50  watts,  or  3.1  watts  per 
candle-power,  with  a  life  of  about  500  hours;  if  the  voltage  is 
increased  2  per  cent.,  the  efficiency  is  increased  about  7  per  cent, 
and  the  life  is  reduced  about  40  per  cent. 

379.  The  Tungsten  Lamp  has  a  filament  of  metallic  tungsten. 
The  temperature  of  such  a  filament  can  be  maintained  at  about 
2000°  C.,  which  is  higher  than  the  operating  temperature  of  a  car- 
bon filament,  so  that  the  tungsten  lamp  is  the  more  efficient,  taking 
only  1.2  watts  per  candle-power,  and  gives  the  whiter  light.     The 

352 


ART.  380] 


ELECTRIC  LIGHTING 


353 


tungsten  lamp  is  the  more  fragile  of  the  two  and  the  life  of  the 
lamp  is  not  limited  by  a  decrease  in  candle-power,  but  by  the 
wear  of  the  filament,  which  causes  it  to  break  after  about  1000 
hours  of  service. 

A  low- voltage  lamp  is  more  robust  than  a  high- voltage  lamp 
of  the  same  candle-power,  because  the  filament  is  shorter  and  of 
larger  cross  section,  since  it  has  to  carry  a  larger  current  with  a 
smaller  voltage  drop. 

One  important  difference  between  tungsten  and  carbon  is  that, 
while  the  temperature  coefficient  of  resistance  of  the  former  is 
positive,  that  of  the  latter  is  negative.  Because  of  this,  the 
tungsten  lamp  is  less  sensitive  than  the  carbon  lamp  to  voltage 
fluctuations.  If  for  example  the  voltage  is  increased  by  k  per 
cent.,  the  corresponding  increase  of  current  in  the  tungsten  lamp 
will  be  less  than  k  per  cent.,  because  the  resistance  of  the  filament 
increases,  while  the  increase  of  current  in  the  carbon  lamp  will  be 
greater  than  k  per  cent,  because  the  resistance  decreases.  The 
effect  of  an  increase  in  .voltage  is  shown  in  the  following  table: 


Voltage 

Candle-power 

Watts  per  candle- 
power 

Life 

Normal  or  100% 
102% 

98% 

100% 
111% 

107% 
90% 
93.3% 

100% 
93% 
96.3% 
106% 
103.7% 

100% 
60%  carbon 
76%  tungsten 
-^1.47%  carbon 

125%  tungsten 

Because  of  the  positive  temperature  coefficient  of  resistance, 
the  tungsten  lamp  has  a  much  lower  resistance  when  cold  than 
when  hot,  so  that,  when  the  lamp  is  switched  on,  the  initial  current 
is  several  times  as  large  as  the  normal  operating  current.  This 
result,  called  overshooting,  reduces  the  life  of  a  lamp  if  it  is 
switched  off  and  on  frequently;  for  sign  lighting,  low- voltage 
lamps  are  used  since  they  have  a  stouter  filament  than  standard 
110- volt  lamps. 

380.  Gas-filled  Tungsten  Lamp. — If  tungsten  is  heated  in  an 
atmosphere  of  nitrogen  instead  of  in  a  vacuum,  the  temperature 
at  which  evaporation  becomes  excessive  is  higher  in  the  former 
case  than  in  the  latter.  Such  nitrogen- filled  lamps  therefore  have 
a  high  efficiency,  and,  in  large  sizes,  take  only  0.5  watts  per  candle- 
power.  Since  the  globe  contains  gas,  convection  currents  are  set 


354     PRINCIPLES  OF  ELECTRICAL  ENGINEERING      [CHAP.  XLII 


up  when  the  lamp  is  lit,  and  the  globe  has  the  comparatively  high 
temperature  of  about  200°  C. 

To  obtain  high  efficiency  from  this  lamp,  it  is  found  that  the 
filament  must  be  large  in  cross  section-;  a  filament  to  carry  20 
amp.  at  10  volts  gives  2  candle-power  per  watt  while  a  thinner 
filament  to  carry  5  amp.  at  40  volts  gives  only  1.4  candle-power 
per  watt.  The  lamp  at  its  best  efficiency  cannot  be  made  at 
present  for  less  than  about  350  candle-power,  in  which  size  it  is 
suitable  for  the  lighting  of  large  areas  at  present  lit  by  arc  lamps 
of  lower  efficiency. 

381.  The  Unit  of  Light. — The  light  giving  power  of  a  lamp  is 
expressed  in  candle  power.  This,  however,  has  little  meaning 


FIG.  422. — Light  distribution  in  a 
vertical  plane,  from  a  32  candle-power 
tungsten  lamp. 


FIG.  423. — One  type  of 
arc-lamp  mechanism. 


unless  the  direction  of  the  light  is  specified,  and  curves  such  a8 
that  in  Fig.  422  are  used  for  this  purpose.  This  curve  shows  the 
distribution  of  light  about  the  vertical  plane  of  a  tungsten  lamp, 
and  is  obtained  by  measuring  the  candle  power  in  different  direc 
tions  and  then  plotting  along  each  radius  a  length  proportional  to 
the  candle  power  in  that  direction. 

Incandescent  lamps  are  generally  rated  in  mean  horizontal 
candle  power.  Thus  the  lamp  on  which  the  curve  in  Fig.  422 
was  taken  would  be  rated  at  32  candle  power  although  the  average 
candle  power  in  all  directions,  called  the  mean  spherical  candle 
power  (m.s.c.p.)  is  only  about  24  m.s.c.p. 

382.  Arc  Lamps. — If  two  sticks  of  carbon  connected  in  an 
electric  circuit  as  shown  in  Fig.  423  are  brought  into  contact,  a 


ART.  384]  ELECTRIC  LIGHTING  355 

current  will  flow  through  the  circuit  and,  since  the  contact  be- 
tween the  carbons  is  poor  and  the  resistance  of  the  contact  is 
therefore  high,  the  carbons  at  the  contact  begin  to  glow,  while  a 
small  quantity  of  carbon  vapor  passes  between  them. 

If  the  carbon  contacts  are  now  separated  by  about  a  quarter  of 
an  inch,  it  will  be  found  that  the  current  still  flows,  because  the 
space  between  the  contacts  is  filled  with  carbon  vapor  which  is 
conducting.  The  arc  so  formed  is  a  powerful  source  of  light. 

An  arc  lamp  consists  of  two  sticks  of  carbon,  with  a  mechanism 
which,  when  the  voltage  is  applied,  brings  the  carbons  into  con- 
tact and  then  separates  them,  and  which  also  feeds  the  carbons 
together  as  they  are  consumed. 

One  of  the  many  types  of  mechanism  for  this  purpose  is  shown 
diagrammatically  in  Fig.  423.  The  upward  pull  of  the  shunt 
coil  S  tends  to  bring  the  carbons  together,  and  this  pull  increases 
with  the  voltage  E;  the  upward  pull  of  the  series  coil  L  increases 
with  the  current  I  and  tends  to  separate  the  carbons.  When  the 
main  switch  is  closed,  a  large  current  I  passes  through  the  coil  L 
while  the  voltage  E  is  comparatively  small,  so  that  the  pull  of  L 
is  greater  than  that  of  S  and  the  carbons  are  separated.  As 
they  separate,  E  increases  and  I  decreases  and,  when  the  arc 
has  reached  the  proper  length,  then  E  and  I  have  their  normal 
values  and  the  pulls  balance. 

383.  The  direct-current  open  arc  takes  the  form  shown  in 
Fig.  425.     The  temperature  of  the  positive  tip  is  about  3700°  C., 
the  temperatures  of  the  arc  stream  and  of  the  negative  tip  are 
much  lower.     Of  the  total  light  from  such  an  arc,  85  per  cent, 
comes  from  the  crater,  10  per  cent,  from  the  positive  tip  and 
5  per  cent,  from  the  arc  stream.     The  distribution  of  light  from 
such  an  arc  is  shown  by  the  polar  curve  in  Fig.  424;  directly  below 
the  arc  the  illumination  is  practically  zero  due  to  the  shadow  cast 
by  the  lower  carbon. 

Because  of  the  high  temperature,  the  light  is  white  and  the 
efficiency  is  high,  but  the  life  of  the  carbons  is  only  about  10  hours. 

384.  Direct-current  Enclosed  Arc.— To  cut  down  the  trimming 
expense,  the  arc  is  enclosed  in  a  globe  which  is  almost  air  tight, 
so  that  after  the  first  few  seconds  the  arc  is  operating  in  an  atmos- 
phere of  carbonic  acid  gas,  and  the  carbons  are  consumed  more 
slowly  than  in  an  open  arc  and  have  a  life  of  about  100  hours. 
The  arc  is  operating  under  slight  pressure  and,  for  satisfactory 
operation,  the  arc  stream  is  longer  than  that  of  the  open  arc. 


356      PRINCIPLES  OF  ELECTRICAL  ENGINEERING      [CHAP.  XLII 


The  crater  is  not  now  so  pronounced,  and  a  greater  portion  of  the 
total  light  now  comes  from  the  arc  stream,  so  that  the  light  dis- 
tribution in  a  horizontal  direction  is  improved.  Data  on  this 
arc  is  given  in  the  table  on  page  360.  . 

385.  Alternating-current  Enclosed  Arc. — When  operating 
with  alternating  current,  the  carbon  arc  lamp  does  not  go  out 
when  the  current  passes  through  the  zero  value,  because  there 
is  enough  heat  in  the  carbon  tips  to  maintain  the  arc  stream  while 
the  current  reverses.  Alternating-current  arcs  have  no  crater, 
and  each  tip  is  equally  hot  but  is  not  so  hot  as  the  crater  of  a 
direct-current  arc,  so  that  the  efficiency  is  lower,  moreover  the 
light  is  not  directed  downward  but  is  directed  horizontally,  so 
that  a  reflector  has-  to  be  supplied  to  deflect  the  light  in  the 
direction  shown  in  Fig.  424. 


FIG.  424. — Light  distribution  of  arc  lamps. 


FIG.  425.— Shape  of  a 
direct-current  arc. 


386.  Flame  Arc  Lamps. — The  positive  carbon  of  the  direct- 
current  flame  arc  lamp  and  either  or  both  carbons  of  the  alter- 
nating current  flame  arc  are  impregnated  with  salts  which  have 
high  selective  radiation.  Such  salts  when  heated  give  most  of 
their  radiation  in  one  particular  part  of  the  spectrum.  Being 
selective,  the  portion  of  the  total  radiation  which  is  given  off  as 
light  is  greater  than  that  given  off  by  an  ordinary  incandescent 
body  at  the  same  temperature,  such  lamps  are  therefore  very 
efficient,  and  a  large  portion  of  the  light  comes  from  the  arc 
stream.  Calcium  salts  are  often  used  and  give  a  yellow  light. 
Barium  salts  give  a  white  light,  but  the  white  flame  arc  is  not-  so 
efficient  as  the  yellow  flame  arc. 


ART.  388]  ELECTRIC  LIGHTING  357 

The  open  flame  arc  requires  daily  trimming.  If  the  arc  is 
enclosed  so  as  to  limit  the  supply  of  air,  the  life  of  the  carbons  may 
be  increased  to  100  hours  without  any  marked  reduction  in  the 
efficiency.  The  enclosing  globe,  however,  must  be  kept  free 
from  soot,  by  arranging  that  the  fumes  shall  be  carried  away  and 
allowed  to  deposit  in  a  condensing  chamber  and  not  on  the  sides 
of  the  enclosing  globe. 

387.  Luminous  Arc   Lamp. — This   is   a  low   temperature  arc 
which  depends  entirely  on  selective  radiation  for  its  efficiency. 
It  is  essentially  a  direct-current  arc  and  has  a  positive  electrode 
of  copper  and  a  negative  electrode  of  magnetite.     Magnetite 
boils  at  a  much  lower  temperature  than  carbon,  and  the  tempera- 
ture of  the  arc  is  not  high  enough  to  melt  the  copper  or  to  enable 
the  arc  to  be  used  on  alternating  current;  the  arc  goes  out  as  the 
current  passes  through  zero. 

The  efficiency  is  not  so  high  as  that  of  the  flame  arc.  The  light 
is  white,  and  the  magnetite  electrode  has  a  life  of  about  150  hours 
while  that  of  the  copper  electrode  exceeds  1000  hours.  The  mag- 
netite arc  must  never  be  connected  with  the  polarity  reversed 
because,  if  the  copper  were  made  the  negative  electrode,  a  copper 
arc  would  be  produced  instead  of  a  magnetite  arc,  since  the  mate- 
rial of  the  arc  stream  comes  from  the  negative  electrode. 

For  alternating- current  circuits,  titanium  arcs  are  being  devel- 
oped, which  have  an  efficiency  comparable  with  that  of  the  flame 
arc. 

388.  Mercury   Vapor   Converter. — To    obtain    direct-current 
from  an  alternating-current  supply  for  the  operation  of  magnetite 
arcs,  the  mercury  vapor  converter  is  used. 

The  globe  shown  in  Fig.  426  is  filled  with  mercury  vapor.  It 
is  found  that  a  very  high  voltage  is  required  to  start  an  arc 
between  a  and  b  but  about  14  volts  is  all  that  is  required  to  main- 
tain the  arc;  there  is  a  high  resistance  at  the  negative  electrode 
which  resistance  is  broken  down  when  current  is  flowing,  but 
is  reestablished  as  soon  as  the  current  ceases  to  flow.  Alter- 
nating current  cannot  pass  between  the  electrodes  because  the 
high  resistance  is  reestablished  as  the  current  passes  through  the 
zero  value. 

To  operate  as  a  converter,  the  globe  is  fitted  with  three  elec- 
trodes and  is  connected  up  as  shown  in  Figi.  428,  and  means  are 
provided  whereby  the  resistance  of  electrode  c  is  kept  broken 
down.  At  the  instant  shown  in  Fig.  428,  the  electrode  a  is  posi- 


358      PRINCIPLES  OF  ELECTRICAL  ENGINEERING      [CHAP.XLII 

tive  and  that  of  b  is  negative,  so  that  current  passes  from  a  to 
c  but  no  current  can  pass  from  a  to  b  or  from  c  to  fr,  because  of  the 
high  resistance  of  negative  electrode  b.  Half  a  cycle  later  b  is 
positive  and  a  is  negative,  so  that  current  can  pass  from  b  to  c  but 
no  current  can  enter  a.  The  current  in  the  line  L  therefore  flows 
always  in  one  direction  or  is  a  direct  current. 

There  is  a  small  reactance  coil  St  called  a  sustaining  coil, 
placed  in  the  line  L  to  carry  the  rectifier  over  the  point  of  zero 
current.  This  coil  causes  the  current  to  lag  slightly  behind  the 
e.m.f.  so  that  there  is  still  a  small  current  flowing  from  a,  forexam- 


Single  Phase  Mains 
Reactance  Coil 


Single  Phase  Mains 


FIG.  426.  FIG.  427.  FIG.  428. 

FIGS.  426  to  428. — The  mercury-vapor  converter. 

pie,  when  the  arc  from  b  is  ready  to  strike.  The  current  entering 
c  therefore  never  becomes  zero  and  the  high  negative  resistance  of 
that  electrode  is  always  broken  down. 

Before  such  a  converter  can  be  started,  the  resistance  at  c  must 
first  be  broken  down.  To  accomplish  this  result  an  additional 
starting  electrode  is  placed  at  w  and  current  is  passed  between 
w  and  c  by  tipping  the  globe  until  the  mercury  forms  a  bridge 
between  these  two  electrodes.  The  tube  is  then  raised  and  an 
arc  is  drawn  between  w  and  c  for  half  a  cycle,  which  breaks  down 
the  resistance  of  c  long  enough  to  allow  the  arc  to  start  from  b 
and  thereby  start  the  rectifier  in  operation. 


ART.  390] 


ELECTRIC  LIGHTING 


359 


389.  Mercury  Vapor  Lamp.— The  converter  described  above 
is  an  arc  lamp  and,  since  the  light  is  due  to  the  selective  radiation 
of  mercury  vapor,  it  is  a  high  efficiency  lamp.  Unfortunately 
the  color  is  greenish  blue  and  gives  ghastly  color  effects. 


Reactance 
Coil 


Reflector 


FIG.  429. — Mercury-vapour  lamp  for  operation  by  alternating-current. 

The  alternating- current  lamp  is  made  in  the  form  shown  in 
Fig.  429,  which  is  lettered  similarly  to  Fig.  427.  To  start  the  arc, 
the  lamp  is  tipped  so  that  the 
mercury  runs  down  and  forms  a 
metallic  connection  between  the 
two  electrodes  through  which  a 
current  flows,  the  lamp  is  then 
allowed  to  return  to  its  original 
position,  the  mercury  thread  is 
ruptured,  and  an  arc  follows. 

The  direct-current  lamp  is 
supplied  with  two  terminals  and 
is  started  in  the  same  way. 

When  a  quartz  tube  is  used 
instead  of  a  glass  tube,  the 
lamp  may  be  shortened,  and  run 
at  a  higher  temperature  with  a 
higher  efficiency. 

390.  Shades  and  Reflectors. 
— The  light  distribution  from  a 
source  can  be  completely  changed 
by  the  use  of  a  shade  or  reflec- 
tor. Curve  A,  Fig.  430,  shows  the  light  distribution  from  a  tung- 
sten lamp  while  curves  B,  C  and  D  show  the  distribution  when 


FIG.  430. — Light  distribution  of 
a  32  candle-power  tungsten  lamp 
equipped  with  different  reflectors. 


360      PRINCIPLES  OF  ELECTRICAL  ENGINEERING      [CHAP.  XLII 


different  types  of  reflectors  are  used.  Curve  B  is  obtained  with 
what  is  called  an  extensive  reflector,  curve  C  with  an  intensive 
reflector,  and  curve  D  with  a  focusing  reflector. 

When  the  distance  between  the  lamps"  is  at  least  twice  the  height 
of  the  lamp  above  the  ground,  the  extensive  type  is  used;  it  is 
suitable  for  a  small  room  lit  by  a  source  placed  on  the  centre  of 
the  ceiling.  When  the  distance  between  lamps  is  about  one  and 
a  half  times  the  mounting  height,  the  intensive  type  is  preferred; 
it  is  suitable  for  a  large  room  lit  from  several  points  on  the  ceiling. 
The  focusing  type  is  used  when  the  lamps  are  more  closely  spaced 
and  is  suitable  for  the  illumination  of  desks,  show  cases,  etc. 

391.  Efficiency  of  Illuminants. — Since  the  light  from  a  source 
can  be  suitably  directed  by  means  of  reflectors,  light  efficiency 
should  be  based  on  the  mean  spherical  candle  power  of  the  lamp. 
Values  attained  in  practice  are  given  below4 


Available 

spherical 
candle 
power  per 
watt 

Candle 
power  at 
10°  angle 
per  watt 

size  in 
mean 
spherical 
candle 

power 

Incandescent  lamps 

Ordinary  carbon  filament 

0.21 

0.4 

any  size 

Tungsten  filament  

0.64 

1.25 

any  size 

Gas  -filled  tungsten                               .    . 

1.28 

2.5 

above  350 

Arc  lamps 

Enclosed  carbon 

6.6  amp.,  450  watts  A.C. 

0.39 

0.5 

175 

6.6  amp.,  480  watts  B.C. 

0.62 

1.0 

300 

Flame  carbon   500  watts,  yellow 

3.1 

6.2 

1550 

300  watts,  yellow 

1.95 

4.0 

585 

500  watts,  white 

1^95 

4.0 

975 

B.C.  magnetite      4  amp.,  300  watts 

1.0 

2.2 

300 

6.6  amp.,  500  watts 

1.5 

3.2 

750 

A.C.  titanium       220  watts 

1.9 

4.0 

420 

Mercury  lamps    glass  tube 

1.55 

quartz  tube 

2.0 

These  figures  take  account  of  the  loss  in  the  reflectors,  and  the 
candle  power  at  10°  angle  below  the  horizontal  is  that  obtained 
when  the  lamp  is  equipped  with  a  reflector  suitable  for  street 
lighting. 

1  Efficiency  of  Ilium  inants  by  C.  P.  Steinmetz,  General  Electric  Review, 
March,  1914. 


AKT.  392] 


ELECTRIC  LIGHTING 


361 


392.  Light  and  Sensation. — The  eye  is  able  to  see  objects 
distinctly  over  a  range  of  intensity  of  1,000,000  to  1  as  determined 
by  the  exposure  of  a  photographic  plate.  The  size  of  the  pupil  is 
controlled  automatically  by  the  intensity  of  the  light  and  decreases 
as  the  light  intensity  increases  so  as  to  limit  the  amount  of  light 
that  can  enter  the  eye.  The  sensibility  of  the  optic  nerve  also 
changes  automatically  with  the  light  intensity  but  at  a  much 
slower  rate;  when  one  goes  from  daylight  into  a  darkened  room, 
for  example,  objects  that  at  first  are  invisible  become  quite 
distinct  after  a  time. 


Ultra  Red 


FIG.  431. — Effect  of  color  on  the  light  efficiency. 

The  light  sensation  produced  by  a  given  amount  of  radiant 
power  entering  the  eye  depends  on  the  color  of  the  light.  White 
light  is  composite  and,  when  passed  through  a  prism,  as  shown  in 
Fig.  431,  is  divided  up  into  its  constituents  and  produces  a  band 
of  color  varying  from  red  to  violet  called  the  spectrum. 

The  relation  between  candle-power  per  watt  and  color  is 
shown  in  Fig.  431;  with  a  high  light  intensity  the  eye  is  most- 
sensitive  to  yellow  light  while  with  a  low  intensity  it  is  most  sen- 
sitive to  green  light.  If  for  example  a  mercury  vapor  lamp  and 
a  yellow  flame  arc  burning  side  by  side  have  the  same  brightness 
at  a  moderate  distance  from  the  observer,  then,  when  the  lamps 
are  cl6se  at  hand  the  flame  arc  appears  the  brighter  since  the  light 

24 


362      PRINCIPLES  OF  ELECTRICAL  ENGINEERING      [CHAP.  XLII 

intensity  is  high,  but  when  the  lamps  are  a  considerable  distance 
away  the  intensity  is  low  and  the  green  mercury  lamp  appears 
the  brighter.  A  yellow  light  is  therefore  the  most  efficient  for 
high  intensity  illumination  and  a  green  light  the  most  efficient 
for  low  intensities ;  these  colors  however  are  often  objectionable. 
There  is  one  important  exception;  yellow  light  is  used  for  fog 
signal  work  because  it  penetrates  fog  better  than  does  green  light. 

393.  Reflection  and  Color. — When  light  strikes  an    opaque 
body,  some  of  the  light  is  absorbed  and  some  is  reflected.     The 
color  of  an  opaque  body  is  the  color  of  the  light  which  it  reflects, 
a  green  opaque  body  is  one  which  absorbs  all  but  the  green  rays. 

When  light  strikes  a  transparent  body,  the  color  of  the  body  as 
seen  from  the  side  away  from  the  source  is  the  color  of  the  light 
which  is  transmitted ;  a  green  glass  transmits  only  the  green  rays, 
whereas  a  transparent  body  transmits  all  the  light  that  falls  on  it, 
a  colored  globe  on  a  lamp  therefore  cuts  off  light  and  reduces  the 
efficiency. 

The  color  of  a  body  depends  on  the  color  of  the  light  that 
strikes  on  it.  An  opaque  body  which  is  yellow  in  daylight  reflects 
only  yellow  rays;  in  the  green  light  of  the  mercury  vapor  lamp 
this  body  looks  black  because  it  absorbs  the  green  light  and  there 
is  no  yellow  to  reflect.  It  is  important  therefore  that  colors  be 
matched  in  the  light  in  which  they  are  going  to  be  used. 

The  color  of  the  light  in  a  room  is  not  always  the  color  of  the 
source.  If  daylight  enters  a  room  which  has  walls  of  a  green  color, 
then  the  bulk  of  the  light  that  strikes  the  walls  is  absorbed  while 
the  green  rays  are  reflected  and  a  green  tint  is  added  to  the  light 
in  the  room.  Dark  walls  and  ceilings  result  in  low  efficiency  of 
illumination.  In  an  ordinary  room,  about  70  per  cent,  of  the 
light  from  the  source  strikes  the  walls  and,  if  the  paper  is  dark, 
most  of  this  light  is  absorbed  and  lost,  if  the  paper  is  light  yellow 
in  color,  a  large  part  of  the  light  is  reflected  and  is  useful. 

Red  wall  papers  and  table  cloths  are  to  be  avoided  in  a  reading 
room  because,  under  such  circumstances,  a  large  part  of  the  light 
that  enters  the  eye  is  red  light  and,  for  a  given  distinctness  of 
vision,  a  larger  amount  of  energy  must  enter  the  eye  if  the  light 
is  red  than  if  yellow  or  white,  see  Fig.  431,  page  361,  and  the 
excessive  amount  of  energy  is  harmful  to  the  eye  tissues. 

394.  Principles  of  Illumination. — For  satisfactory  illumination, 
the  light  should  be  of  good  quality,  glare  should  be  avoided,  and 
the  shadows  should  be  distinct  so  as  to  give  good  perspective. 


ART.  397]  ELECTRIC  LIGHTING  363 

395.  Quality  of  the  Light.— The  light  should  be  as  nearly  white 
as  possible.     Most  of  the  harm  done  by  artificial  illumination  is 
due  to  the  red  and  ultra  red  rays  for  the  reason  just  pointed  out. 
Green  light  also  is  harmful  under  certain  circumstances  because 
it  is  found  that  the  pupil  fails  to  respond  to  variations  in  intensity 
of  light  of  this  color.     Green  light  should  therefore  not  be  used 
for  high  intensity  illumination,  as  for  example  for  the  lighting  of 
drawing  offices,  but,  as  pointed  out  on  page  362,  it  is  particularly 
suited  for  low  intensity  illumination.     Ultra  red  and  ultra  violet 
radiation  is  the  most  harmful  and  there  is  much  of  the  latter  in 
arc  lamps,  so  that  an  arc  should  always  be  enclosed  in  a  glass 
globe  since  glass  is  opaque  to  such  radiation. 

Flickering  light  is  bad  for  the  eye  because  the  pupil  tries  to 
adjust  itself  to  the  rapidly  varying  intensity  and  becomes  fatigued. 
Alternating  current  lighting  at  25  cycles  has  not  been  uniformly 
successful  for  this  reason. 

396.  Glare. — The  eye  cannot  look  with  comfort  on  objects 
which  have  a  higher  surface  intensity  than  4  candle  power  per 
square  inch,  so  that  the  sources  of  illumination  should  be  kept 
out  of  the  range  of  vision,  and  direct  reflection  into  the  eye  from 
such  sources  should  also  be  avoided.     Side  lights  and  light  from 
below  are  particularly  objectionable  since  the  eye  is  not  protected 
from  such  light. 

If  the  source  of  illumination  cannot  be  kept  out  of  the  range 
of  vision,  then  frosted  incandescent  lamps  should  be  used  or  the 
lamp  supplied  with  a  shade  or  globe  that  will  keep  the  direct 
rays  away  from  the  eye. 

It  is  impossible  to  see  distinctly  past  a  bright  light  and  for 
that  reason  country  roads  should  not  be  lit  by  powerful  arc  lamps 
spaced  far  apart  because,  not  only  does  most  of  the  light  go  to 
illuminate  the  adjoining  fields,  but  the  arcs  are  generally  hung 
so  low  to  clear  the  trees  that  it  is  impossible  to  see  past  them,  and 
driving  on  such  roads  is  dangerous.  Incandescent  lamps  of 
moderate  candle  power,  spaced  closer  together,  give  better  illu- 
mination. 

397.  Shadows. — In  certain  cases  as  for  example  for  the  illu- 
mination of  drafting  rooms  it  is  desirable  to  eliminate  shadows. 
This  result  is  obtained  by  the  use  of  a  large  number  of  light 
sources,  or  by  the  use  of  indirect  methods  of  lighting  whereby 
a  [reflector  is  used  to  throw  all  the  light  on  to  the  ceiling  from 
which  it  is  reflected  on  to  the  drawing  tables. 


364      PRINCIPLES  OF  ELECTRICAL  ENGINEERING      [CHAP.  XLII 

For  other  classes  of  work,  such  as  street  lighting,  diffusion  is  to 
be  avoided  since,  due  to  the  elimination  of  shadows,  there  is  loss 
of  perspective  and  obstacles  are  not  clearly  seen. 

398.  Intensity  of  Illumination. — The  unit  of  light  intensity 
is  that  produced  by  a  source  of  one  candle-power  at  a  distance  of 
1  ft.  and  is  called  the  foot-candle. 

In  Fig.  432,  the  same  amount  of  light  strikes  the  surfaces 
A,  B  and  C,  so  that  the  intensity  at  B  is  less  than  that  at  A  in 
the  ratio  ar2/6r2,  or  intensity  is  inversely  proportional  to  the 
square  of  the  distance.  Again  the  intensity  on  surface  C  is  less 
than  that  at  B  in  the  ratio  area  B/  area  C  =  cos  a. 


O      10         20        30         40         50   Candle  Power 


FIG.  432. — Variations  of  light  intensity  with  the  distance  from  the  source, 
and  with  the  angle  of  incidence. 

Curve  D,  Fig.  432,  shows  the  light  distribution  curve  of  a  40-candle- 
power  tungsten  lamp  equipped  with  a  reflector  for  street  lighting.  It  is  re- 
quired to  determine  the  illumination  at  a  point  X,  50ft.  from  the  post,  the 
height  of  the  lamp  being  12  ft. 

The  candle-power  in  direction  OX  =  51 

The  distance  OX  =  \/50H-  122    =  51.5  ft. 

The  intensity  on  an  obstacle  normal  to  the  light 

=  Tgi  5y2  =  0.0192  ft.-candle 
The  intensity  on  the  street  surface  =  0.0192  X  cos  a 

=  0.0192  X  42C  =  0.0045  ft.-candles. 
51.5 

The  minimum  intensity  on  the  street  surface  due  to  two  lamps  spaced 
100  ft.  apart  =  2  X  0.0045  =  0.009  ft.-candles.  For  country  roads,  the 
minimum  intensity  on  an  obstacle  normal  to  the  light  should  not  be  less 
than  0.02  ft.-candles. 

399.  Lines  of  Illumination. — It  is  convenient  to  represent  the 
total  light  from  a  source  by  lines  of  illumination,  called  lumens, 


ART.  400]  ELECTRIC  LIGHTING  365 

such  that  the  number  crossing  unit  area  placed  perpendicular  to 
the  direction  of  the  light  is  made  proportional  to  the  light  in- 
tensity. One  foot-candle  is  represented  by  one  line  per  sq.  ft. 

If  a  source  of  one  candle-power  were  surrounded  by  a  sphere 
of  1  ft.  radius,  the  intensity  at  the  surface  of  the  sphere 
would  be  1  ft.-candle,  there  must  therefore  be  one  line  per  sq. 
ft.  or  a  total  of  4?r  lines. 

To  provide  for  a  surface  intensity  of  /  ft.-canclles  over  an  area 
of  A  sq.  ft. 

The  number  of  lines  of  illumination  =  /  X  A 

I  X  A 

The  candle-power  of  the  source        =  — -r— 

rrTT 

Since  some  of  the  light  is  absorbed  by  walls  and  ceilings 

/  X  A 

the  necessary  candle-power  =  •-: — — r- 

4rr  X  K> 

useful  light  . 

where  k  =  -  -  is  less  than  1 :  average  values  are 

total  light 

k  =  0.6  if  clear  reflectors  are  used  and  the  room  has  light 

walls  and  ceiling. 
=  0.4  with  clear  reflectors  and  dark  walls  and  ceiling. 

Determine  the  candle-power  required  to  give  a  light  intensity  of  3  ft.- 
candles  over  a  room  which  is  40  ft.  long  and  30  ft.  wide  the  room  having 
light  walls  and  ceiling.1 

The  necessary  candle-power  =  —       ^7n«"     =  477    candle-power.     A  40- 

TcTT    /\  U.O 

watt    lamp    gives    40/1.25  =  32    horizontal    candle-power   and    24   mean 

477 
spherical  candle-power,  see  page  354,  so  that    ^ .    =  20  lamps  of  40  watts 

each  are  required,  or  a  smaller  number  of  lamps  of  larger  candle-power.     The 
choice  of  the  candle-power  of  each  lamp,  and  the  spacing  of  the  lamps,  is  a 
matter  that  must  be  left  to  the  judgment  of  the  individual;  there  are  many 
examples  of  good  and  of  bad  illumination  to  be  found  in  every  city. 
In  the  above  problem 

The  total  power  required  =  20  X  40  =  800  watts 

The  watts  per  sq.  ft.  =  800/(40  X  30)  =  0.75. 

400.  Power  Distribution  for  Lighting. — Interior  lighting  is 
generally  carried  out  at  110  volts,  either  alternating  or  direct 
current,  the  lamps  being  connected  in  parallel  across  the  circuit. 
When  arc  lamps  are  connected  across  such  a  circuit,  a  steady- 
ing resistance  must  be  placed  in  series  with  the  arc  as  shown  in  Fig. 
433.  Suppose  an  arc  is  carrying  a  current  7  with  a  constant 

1  A  list  of  economical  intensities  for  all  classes  of  work  will  be  found  in 
the  American  Electricians'  Handbook  by  Terrell  Croft. 


366     PRINCIPLES  OF  ELECTRICAL  ENGINEERING      [CHAP.  XLII 


voltage  E  at  the  terminals.  If  the  current  7  were  to  increase  for 
an  instant,  more  carbon  vapor  would  leave  the  negative  electrode, 
the  arc  stream  would  become  more  conducting,  and  the  current 
I  would  increase  still  further,  so  that  an  a'rc  is  unstable  when  used 


1.  ft         1 

• 


Constant 


Constant 


PIG.  433. — Direct-current.          FIG.  434. — Alternating-current. 
FIGS.  433  and  434. — Multiple  arcs. 

on  constant  potential.  If,  however,  a  resistance  is  placed  in 
series  with  the  arc,  as  shown  in  Fig.  433,  then  an  increase  in  the 
current  /  causes  the  voltage  E\  =  IR  to  increase,  and  therefore 
the  arc  voltage  E2  to  decrease  so  that  it  cannot  maintain  the  in- 
creased current  across  the  arc. 


Constant  Current 
Transformer 


FIG.  435. — Alternating-current  system. 


Mercury  Vapour 
Converter 

FIG.  436. — Direct-current  system. 
Series  connection  of  lamps. 

In  the  case  of  alternating- current  parallel  arcs  a  steadying 
reactance  is  used,  because  it  consumes  very  little  power  and  still 
reduces  the  voltage.  The  same  result  may  be  obtained  by  sup- 
plying the  lamp  through  an  autotransformer  since  the  sec- 
ondary voltage  #2;  Fig.  434,  decreases  with  an  increase  of  the  arc 
current  /2. 

If  the  lamps  can  be  used  in  a  series  circuit,  as  shown  in  Fig. 
435,  then  the  wire  has  to  carry  the  current  of  only  one  lamp  and 


AIIT.  400]  ELECTRIC  LIGHTING  367 

may  be  small  in  cross  section.  This  system  of  distribution  is 
largely  used  for  street  lighting,  the  constant  current  required  for 
the  operation  of  the  arcs  being  obtained  by  means  of  a  constant 
current  transformer,  see  page  267. 

By  means  of  a  series  transformer  s  it  is  possible  to  connect  a 
circuit  of  tungsten  lamps  taking  a  large  current  in  series  with  a 
main  circuit  of  arc  lamps  taking  a  smaller  current. 

For  the  operation  of  magnetite  arcs,  direct  current  is  necessary 
and  this  is  obtained  by  means  of  a  mercury  vapor  converter  con- 
nected as  shown  in  Fig.  436. 

The  voltage  between  electrodes  should  be  about: 

47  volts  for  an  open  carbon  arc, 

72  volts  for  an  enclosed  carbon  arc, 

45  volts  for  an  open  flame  arc, 

78  volts  for  open  magnetite  arc. 


CHAPTER  XLIII 
LABORATORY  COURSE 

401.  Protection  of  Circuits. — A  typical  circuit  is  shown  in 
Fig.  437.  The  lamps  L  take  power  from  the  mains  and  the 
current  I  is  measured  by  the  ammeter  A  while  the  voltage  E  is 
measured  by  the  voltmeter  V,  see  page  19. 

All  the  connections  should  be  made  before  the  switch  S  is 
closed,  and  the  circuit  must  be  protected  by  fuses  F,  see  page  117, 
or  by  an  automatic  circuit  breaker,  see  page  39,  set  so  as  to 
open  the  circuit  if  the  current  should  become  large  enough  to 
injure  any  of  the  apparatus  connected  in  the  circuit. 

To  make  sure  that  the  instruments  are  reading  in  the  proper 
direction,  the  switch  S  should  be  closed  for  an  instant  and  then 


Ammeter 
E    fay         Shunt  A  <\   A 

i  I  _  LJ 


FIG.  437.  —  Typical  circuit. 


opened  before  any  appreciable  current  has  had  time  to  flow. 
If  the  needle  moves  in  the  wrong  direction,  the  instrument 
connections  must  be  reversed. 

402.  Ammeter    Shunts.  —  Most   ammeters    are   wound    with 
very  fine  wire,  see  page  8,  and  can  carry  only  a  small  fraction 
of  an  ampere  without  being  burnt  out.     To  use  such  instruments 
for  the  measurement  of  large  currents  they  must  be  connected 
in  parallel  with  shunts  as  shown  in  Fig.  437.     The  current  in 
the  instrument,  and  therefore  the  deflection,  will  be  proportional  to 
the  line  current  I  so  that,  if  the  shunt  and  instrument  are  always 
used  together,  the  scale  of  the  instrument  may  be  in  terms  of  the 
line  current  to  be  measured  and  not  in  terms  of  the  current  in  the 
instrument.     For  instruments  with  a  range  of  less  than  5  am- 
peres, the  shunt  is  generally  placed  inside  the  case. 

403.  Safe  Carrying  Capacity  of  Copper  Wires.  —  If  too  large 
a   current  flows   in   an   insulated  wire,  the  insulation  will  be 

368 


ART.  404] 


LABORATORY  COURSE 


369 


damaged.     The  values  given  in  the  following  table  should  not 
be  exceeded. 


Size  of  wire,  Brown1  ~. 
P     0,                           Diameter  of  wire 
&    Sharpe     gauge            .     .     , 
in  inches 
number 

; 

Maximum  current  in  the  wire  in 
amperes 

Rubber 
insulation 

Other 
insulation 

14 

0.064 

12 

16 

12 

0.081 

17 

23 

10 

0.102 

24 

32 

8 

0.128 

33 

46 

6 

0.162 

46 

65 

5 

0.182 

54 

77 

4 

0.204 

65 

92 

3 

0.229 

76 

110 

2 

0.258 

90 

131 

1 

0.289 

107 

156 

0 

0.325 

127 

185 

404.  Control  of  the  Current  in  a  Circuit. — To  vary  the  current 
flowing  in  the  coil  C,  Fig.  438,  an  adjustable  resistance  R  is 
inserted  in  the  circuit,  see  page  25.  This  rheostat  must  be 
able  to  carry  the  current  I  without  overheating. 


— *— AAAA/WV 
R 


FIG.  438.  FIG.  439. 

FIGS.  438  and  439. — Methods  of  controlling  the  current  in  a  circuit. 

If  the  resistance  of  the  coil  C  is  large  compared  with  that  of 
the  resistance  R,  then  the  current  variation  will  not  be  very 
large.  Under  such  circumstances,  the  potentiometer^connection 
shown  in  Fig.  439  is  to  be  preferred  for  experimental  work.  If 
the  movable  contact  a  is  placed  at  b  then  the  voltage  EI  is  equal 
to  E,  the  line  voltage.  If  contact  a  is  placed  at  c,  then  the 
voltage  EI  is  zero.  By  moving  the  contact  between  these  two 
points,  the  voltage  EI  and  the  current  /may  have  any  value  from 
zero  to  EI  =  E  and  I  =  E/RC.  If  the  resistance  Rc  is  small  and 
the  contact  a  is  close  to  6,  as  shown  in  Fig.  439,  then  the  current 
in  ab  may  become  dangerously  large,  so  that  the  rheostat  must 


Voltmet 


Field 


370     PRINCIPLES  OF  ELECTRICAL  ENGINEERING     [CHAP.  XLIII 

be   watched   during   operation   and   the   circuit   opened   if  the 
rheostat  becomes  too  hot. 

EXPERIMENT  1, 

Object  of  Experiment.  —  To  determine  the  resistance  of  the 
shunt  field  circuit  of  a  direct-current  machine  with  different 
values  of  current  in  the  circuit. 

Reference.—  Pages  20  and  95. 

Connections.  —  If  the  connection  in  Fig.  440  does  not  give 
sufficient  range  of  current,  then  use  the  connection  shown  in 
Fig.  439. 

Readings.  —  Volts  and  amperes. 

Report.  —  Describe     the     method     of 

Ammeter  .  ,  .  ,  .         , 

carrying  out  the  experiment  and  embody 
^e  answers  ^°  ^ne  following  questions  in 
the  report.' 

1.  Find  the  resistance  of  the  field  coil 
circuit. 

FIG.  440.  2.  How  does  temperature  affect  this 

resistance?     Show    the    result     experi- 

mentally  by   measuring   the   resistance   after   the  current  has 
been  flowing  for  30  minutes. 

3.  What  per  cent,  of  the  output  of  the  machine  is  the  excitation 
loss?     The  machine  output  is  given  on  the  name  plate. 

4.  What  range  of  instruments  would  you  use  to   determine 
the  resistance  of  the  shunt  field  circuit  of  a  50  kw.,  240   volt 
generator  if  the  excitation  loss  is  known  to  be  less  than  4  per 
cent.? 

EXPERIMENT  2 

Object  of  Experiment.  —  To  determine  the  resistance  of  the 
armature  circuit  of  a  direct-current  machine  with  different  values 
of  current  in  the  circuit. 

Connections.  —  See.  Fig.  438,  page  369.  The  rheostat  used 
to  control  the  current  must  be  able  to  carry  the  full-load  armature 
current  of  the  machine  without  injury. 

Readings.  —  Volts  across  the  terminals;  volts  across  the  com- 
mutator, obtained  by  attaching  the  voltmeter  leads  to  the  seg- 
ments which  are  under  the  brushes;  amperes. 

Report.  —  Describe  the  method  of  carrying  out  the  experiment 
and  embody  the  answers  to  the  following  questions  in  the  report  : 


ART.  404]  LABORATORY  COURSE  371 

1.  Plot  the  armature  resistance  against  current. 

2.  Plot  the  brush  contact  resistance  against  current. 

3.  What  per  cent,  of  the  output  of  the  machine  are  the  armature 
resistance  loss  and  the  brush  contact  resistance  loss  at  full-load? 

4.  Has  the  pressure  on  the  brushes  much  effect  on  the  brush 
contact  resistance?  try  the  experiment.     What  do  you  imagine 
limits  the  brush  pressure? 

5.  What  range  of  instruments  would  you  use  to  determine  the 
resistance  of  the  armature  circuit  of  a  50-kw.,  240- volt  generator  if 
the  loss  in  the  complete  armature  circuit  is  known  to  be  less  than 
4  per  cent,  at  full-load? 

EXPERIMENT  3 

Object  of  Experiment. — To  find  how  the  speed  of  a  direct 
current  shunt  motor  at  no-load  varies  with: 

a.  The  exciting  current;  armature  voltage  being  constant. 

b.  The  armature  voltage;  exciting  cur- 
rent being  constant. 

References. — Pages  85  and  89. 

Connection.     Fig.  441. 

Readings. — Exciting    current;   arma- 
ture voltage;  speed.  pIG  441 

Curves  a.  Speed  and  exciting  current. 
b.  Speed  and  armature  voltage. 

Questions. — 1.  Explain   the   shape    of   the    curves,    without 
formulae 

2.  What  would  happen  if,  in  starting  up  a  shunt  motor,  a 
starting  resistance  was  not  placed  in  series  with  the  armature 
circuit?     How  many  times  full-load  current  would  flow  through 
the  armature  under  these  conditions?    (Use  the  value  of  armature 
resistance  determined  in  the  last  experiment.) 

3.  What  is  the  back  e.m.f .  of  a  motor  and  what  relation  has  it  to 
the  applied  e.m.f.?     Perform  the  experiment  described  in  Art. 
88,  page  79,  to  answer  this  question. 

4.  What  would  happen    if,   during  operation,  the  field  coil 
circuit  were  to  open  (do  not  perform  this  experiment)  ? 

5.  Draw  a  diagram  of  connections  of  any  starter  in  the  labo- 
ratory, which  has  a  no-voltage  release  and  also  overload  pro- 
tection. 

EXPERIMENT  4 

Object  of  Experiment. — To  find  how  the  voltage  of  a  direct- 
current  generator  at  no-load  varies  with : 


372     PRINCIPLES  OF  ELECTRICAL  ENGINEERING     [CHAP.  XLIII 

a.  The  exciting  current;  speed  being  constant. 

b.  The  speed;  exciting  current  being  constant. 
Reference. — Page  70. 

Connection. — See  Fig.  95,  page  70. 

Readings. — Voltage,  exciting  current  and  speed. 

Curves  a.  Voltage  and  exciting  current. 

b.  Voltage  and  speed. 

Questions. — 1.  Why  is  there  a  small  voltage  even  with  no 
exciting  current? 

2.  Explain  the  shape  of  the  curves. 

3.  How  would  you  reverse  the  polarity  of  the  generator,  i.e., 
how  reverse  the  direction  of  the  voltage? 

EXPERIMENT  5 

Object  of  Experiment. — To  determine  how  the  terminal  volt- 
age of  a  constant  speed  generator  varies  with  the  load : 

a.  The  generator  being  separately  excited; 

b.  The  generator  being  shunt  excited; 

c.  The  generator  being  compound  excited. 
References. — Pages  71  to  77. 

Connections.— See  Fig.  97,  page  72;  Fig.  98,  Fig.  99. 

Readings. — Terminal  voltage,  line  current,  exciting  current 
and  speed. 

Curves. — Terminal  voltage  on  a  line  current  base. 

Shunt  exciting  current  on  a  line  current  base. 

Questions. — 1.  Why  does  the  terminal  voltage  of  a  separately 
excited  generator  decrease  with  increase  of  load?  How  much  of 
the  drop  at  full-load  is  due  to  armature  resistance? 

2.  Why  is  the  voltage  drop  greater  in  a  shunt  than  in  a  sepa- 
rately excited  generator? 

3.  What  would  be  the  effect  of  shifting  the  brushes  further  for- 
ward from  the  neutral  position? 

4.  What  is  the  principal  advantage  of  the  compound  generator?. 

5.  How  would  the  terminal  voltage  of  a  compound  generator 
vary  with  increase  of  load  if  the  series  field  coils  were  connected 
so  as  to  oppose  the  shunt  coils? 

6.  If  the  generator  is  overcompounded  while  flat  compounding 
is  desired,  what  can  be  done  to  fix  the  machine? 

7.  If  the  voltage  of  a  shunt  generator  builds  up  when  the 
generator  rotates  in  a  given  direction,  why  will  it  not  build  up  if 
the  direction  of  rotation  is  reversed?     Try  the  experiment. 


ART.  404]  LABORATORY  COURSE  373 

EXPERIMENT  6 

Object  of  Experiment. — To  determine  the  efficiency  and  also 
the  speed  and  torque  characteristics  of  shunt,  series,  and  com- 
pound motors,  by  loading  the  machines  by  means  of  a  brake, 
the  applied  voltage  being  constant. 

References.— Chaps.  XV,  XVII  and  XVIII. 

Connections.— Fig.  107,  page  86,  Fig.  113  and  Fig.  117. 

Readings. — Applied  voltage  (constant),  armature  current, 
shunt  exciting  current  (constant),  speed,  brake  reading. 

Curves. — Speed  and  torque  on  an  armature  current  base. 
Brake  horse-power  and  total  input  on  an  armature  current  base. 
Efficiency  on  a  brake  horse-power  base. 

Questions. — 1.  How  would  you  reverse  the  direction  of 
rotation  of  each  machine? 

2.  For  what  type  of  service  is  each  machine  suited? 

3.  The  field  coils  of  a  machine  become  hot  during  operation, 
what  effect  will  this  have  on  the  no-load  and  on  the  full-load 
speed  of  each  type  of  motor? 

4.  How  can  the  speed  of  each  type  of  motor  be  varied  for  a 
given  load? 

5.  Why  is  the  speed  regulation  of  a  shunt  motor  poor  when 
the  speed  is  controlled  by  a  resistance  in  the  armature  circuit? 
Try  the  experiment. 

6.  Explain  without  formulae,  the  shapes  of  the  speed,  torque, 
and  efficiency  curves. 

EXPERIMENT  7 

Object  of  Experiment. — To  determine  the  relation  between 
starting  torque  and  armature  current  in  a  shunt,  a  series,  and  a 
compound  motor. 

References. —Pages  85  and  90. 

Connections. — Same  as  for  experiment  6,  with  resistance  in 
the  armature  circuit  to  limit  the  current  (do  not  use  a  starting 
box  for  this  purpose). 

Readings. — Armature  current  and  brake  reading. 

Curves. — Torque  on  an  armature  current  base. 

Questions. — 1.  How  does  the  starting  torque  compare  with 
the  running  torque,  as  determined  in  experiment  6,  for  the  same 
armature  current?  Does  theory  indicate  that  there  should  be  a 
difference? 

2.  Why  is  the  series  motor  preferred  for  heavy  starting  duty? 


374     PRINCIPLES  OF  ELECTRICAL  ENGINEERING     [CHAP.  XLIII 

EXPERIMENT  8 

Object  of  Experiment. — To  measure  the  stray  loss,  the 
armature  copper  loss,  and  the  excitation  loss  in  a  shunt  motor, 
and  calculate  the  efficiency  from  these  figures. 

Reference. — Chap.  XVI,  page  95. 

The  work  of  this  experiment  should  be  done  without  instruc- 
tion. A  diagram  of  connections  should  be  drawn  out  and  the 
range  of  the  necessary  instruments  determined  before  the 
apparatus  is  connected  up. 

Report. — Describe  the  method  of  carrying  out  the  experi- 
ment. Plot  the  efficiency  curve  on  a  horse-power  output  base 
up  to  25  per  cent,  overload,  and  compare  this  curve  with  that 
obtained  by  brake  readings  in  experiment  6;  if  the  results  show 
considerable  difference,  which  would  you  consider  to  be  the  more 
reliable? 

EXPERIMENT  9 

Object  of  Experiment.  To  run  a  shunt  generator  in  parallel 
with  the  power  house  and  determine  the  temperature  rise  of  the 
machine  at  full-load. 

References.— Pages  21,  99  and  163. 

Connection. — Fig.  179,  page  164. 

Readings. — Measure  the  resistance  of  the  field  coil  circuit 
at  the  beginning  of  the  test  and  every  ten  minutes  thereafter,  take 
also  readings  of  temperature  of  the  field  coil  surface  at  the  same 
time.  Find  the  temperature  of  the  armature  core  and  armature 
winding  immediately  the  generator  is  shut  down;  the  heat  run 
should  last  for  at  least  an  hour  and  a  half. 

Curves. — Observed  temperature  rise  of  field  coil  surface ,  and 
also  the  temperature  rise  determined  by  resistance  measurements, 
on  a  time  base. 

Questions. — 1.  Explain  the  shape  of  the  curves. 

2.  Why  is  the  temperature  rise  of  the  field  coils  as  determined 
by  resistance  measurements  greater  than  that  determined  by 
thermometer? 

EXPERIMENT  10 

Object  of  Experiment. — To  determine  the  voltage  regulation 
of  a  three  wire  system. 

References.— Pages  316,  317  and  338. 

Connections. — Use  a  balancer  set,  see  Fig.  374,  or  a  three- wire 


Rotary 
onvert 


ART.  404]  LABORATORY  COURSE  375 

generator.  If  these  are  not  available,  a  single-phase  rotary 
converter  with  a  suitable  autotransformer  may  be  connected 
up  as  shown  in  Fig.  442  to  form  a  three-wire  generator. 

Readings. — Vary  the  loads  on  the  two  sides  of  the  system  from 
perfect  balance,  when  AI  =  A2  and  An  is  zero,  to  a  maximum 
of  unbalance  when  the  load  on  one  side  of  the  line  is  zero.  Take 
readings  of  Vi,  V2,  AI,  and  A2',  V\  +  V2  to  be  kept  constant. 

Curves. — Plot  V\  and    V%  against 
the  unbalanced  current  Ai~A2. 

Questions. — 1.  Explain  the  action 
of  the  apparatus  used. 

2.  What  are  the  advantages  of  the 
three-wire  over  the  two- wire  system 

of  distribution?  An_ 

3.  Explain  the  shapes  of  the  curves.   .  FIG.  442. 

EXPERIMENT  11 

Object  of  Experiment. — To  determine  the  characteristics  of 
fuse  wire. 

Reference.    Page  117. 
Connection.     Fig.  443. 

Readings. — Length  of  wire  between  blocks,  average  current, 
time  taken  for  fuse  to  melt  after  switch  S  is  closed. 

ruse  wire  Curves. — Plot  amperes  on  a  time 
base  for  four  lengths  of  fuse  wire  and 
from  these  curves  plot  another  set 
with  amperes  on  a  length  base  for  a 


FIG   443  fusing  time  of  10  sec. 

Questions. — 1.  Explain  the  shape 
of  the  curves. 

2.  If  a  fuse  is  rated  at  5  amp.,  about  what  current  would  you 
expect  it  to  carry  continuously,  and  what  current  for  30  sec.  ? 

EXPERIMENT  12 

Object  of  Experiment. — To  calibrate  a  circuit  breaker. 
Reference.     Page  39. 
Connections. — Same  as  for  fuse  testing. 
Readings. — Amperes  to  open,  and  position  of  plunger. 
Curve. — Draw  a  current  scale  that  could  be  attached  to  the 
circuit  breaker. 


376     PRINCIPLES  OF  ELECTRICAL  ENGINEERING     [CHAP.  XLIII 

Questions. — 1.  Explain  the  construction  and  the  operation  of 
the  circuit  breaker  used. 

2.  What   are   the   advantages   and   disadvantages   of   circuit 
breakers  and  fuses,  for  what  type  of  circuit  is  each  suited? 

3.  If  a  10  amp.  fuse,  and  a  circuit  breaker  set  for  15  amp., 
are  used  to  protect  the  same  circuit,  which  would  open  first  in 
the  case  of  an  overload  on  the  circuit. 

4.  If  a  circuit  breaker  and  a  switch  are  both  in  a  circuit,  as  in 
Fig.  417,  page  349,  which  would  you  close  first? 

EXPERIMENT  13 

Object  of  Experiment. — To  determine  the  effect  of  change  of 
frequency  on  the  constants  of  an  alternating- current  circuit. 
Reference.     Chaps.  XXIX  and  XXX. 
Connection.     Fig.  444. 

Readings. — Vary  the  alternator  speed  and  adjust  the  alter- 
nator excitation  to  keep  Et  con- 
stant. Take  readings  of  Er,  Et, 
Ec,  I,  watts,  and  speed.  Measure 
the  resistance  R  and  the  resistance 
of  the  coil  Xi  with  direct  current. 

Curves. — On  a  frequency  base, 
plot  the  inductive  reactance  Xi  = 
Ei/I,    the    capacity   reactance    X2  =  EC/I,   the    resistance  R, 

/  W 
and  the  power  factor  I  ^  T 

yM 

Questions. — 1.  For  any  one  frequency,  draw  the  voltage  vector 
diagram  to  scale,  and  compare  the  value  of  Et  so  found  with  that 
found  experimentally. 

2.  Calculate  the  coefficient  of  self  induction  of  the  coil  Xi  and 
the  capacity  of  the  condenser  Xz. 

3.  What  is  meant  by  resonance?     At  what  frequency  does  it 
occur  in  this  experiment  and  how  does  that  frequency  compare 

with  the  theoretical  value /  =  .  --.       ? 

4.  Explain  the  shape  of  the  power  factor  curve. 

5.  Under  what  conditions  can  the  voltage  EI  be  greater  than 
the  applied  voltage  Et! 

EXPERIMENT  14 

Object  of  Experiment. — To  predetermine  the  characteristics 
of  a  given  circuit  and  compare  the  results  with  those  obtained  by 
actual  test. 


ART.  404]  LABORATORY  COURSE  377 

Reference.     Chaps.  XXIX  and  XXX. 

Connection.  —  Use  the  same  resistance,  inductance,  and  capacity 
as  in  the  last  experiment  and  connect  as  in  Fig.  445. 

Curves.—  Predetermine  the  values  of  Er,  EX)  Ih  Ic  and  /, 
with  EtJ  the  normal  voltage  of  the  alternator,  the  same  as  in  the 
last  experiment.  Plot  these  values  Xi 

against  frequency,  then  determine  the 
same  curves  by  test  and  compare  the 
results. 


-EV 
EXPERIMENT  15  -* 


FIG.  445. 
Object  of  Experiment.  —  To  deter- 

mine the  characteristics  of  a  transformer. 
References.     Chap.  XXXIV,  page  261. 

Connections.  —  A  low  voltage  transformer  should  be  used,  con- 
nected as  shown  in  Fig.  446,  or  else  two  like  transformers  con- 
nected as  shown  in  Fig.  447  with  the  high-  voltage  leads  taped  up. 
Readings.  —  Keep  EI,  the  secondary  power  factor  W»/E2I2, 
and   the  frequency   all    constant,  and 
take  readings  of  E\,  I\,  TFi,  E2,  72,  and 
Wz  for  values  of  72  from  zero  up  to  25 
per  cent,  over-load.     Measure  also  the 
FIG.  447.     resistances  of  the  primary  and  the  sec- 
ondary windings  with  direct  current. 

Curves.  —  Plot  efficiency  (W2/Wi)}  voltage  V2,  and  primary  and 
secondary  power  factors  against  72. 

Questions.  —  1.  Why  is  the  primary  power  factor  less  than  that 
of  the  secondary  particularly  at  light  loads? 

2.  Why  does  the  voltage  E2  decrease  with  increase  of  load,  and 
why  is  the  voltage  drop  greater  with  inductive  than  with  non- 
inductive  load? 

3.  Calculate   the  resistance  loss  in  the  windings  at  full-load. 
Find  also  by  calculation  the  full-load  efficiency  and  compare  the 
result  with  that  determined  by  test. 

4.  If  the  transformer  is  connected  to  the  line  for  24  hours  a 
day,  but  carries  full-load  for  only  5  hours  a  day,  find  the  all  day 
efficiency. 

EXPERIMENT  16 

Object  of  Experiment.  —  To  predetermine  the  regulation  curves 
of  a  single-phase  alternator  at  100  per  cent.,  80  per  cent,  and 

25 


378     PRINCIPLES  OF  ELECTRICAL  ENGINEERING     [CHAP.  XLIII 

zero  power  factors,  from  no-load  saturation  and  short-circuit 
curves,  and  compare  the  result  at  100  per  cent,  power  factor 
with  that  found  by  actual  load  test.  (A  lamp  bank  has  a  power 
factor  of  approximately  100  per  cent.) 

Reference.    XXXII,  page  244. 

Connections.     Fig.  287,  page  247. 

Readings. — a.  No-load  saturation;  armature  volts  and  field 
current  at  constant  speed. 

b.  Short-circuit;  armature  amperes  and  field  current  at  the 
same  speed. 

c.  Load  curve;  terminal  voltage  and  armature  current  with 
constant  field  excitation  and  the  same  constant  speed. 

d.  Measure  the  armature  resistance  with  direct  current. 
Curves. — a.  No-load  saturation;  armature  voltage  on  a  field 

current  base. 

b.  Short  circuit;  armature  current  on  a  field  current  base. 

c.  Armature  reactance  determined  from  curves  a  and  b  plotted 
on  a  field  current  base. 

d.  Load  curve;  terminal  voltage  on  an  armature  current  base, 
by  calculation  at  100  per  cent.,  80  per  cent,   and  zero  power 
factors,  and  by  test  at  100  per  cent,  power  factor. 

Questions. — 1.  Why  is  the  power  factor  of  a  bank  of  in- 
candescent lamps  approximately  equal  to  100  per  cent. 

2.  Give  the  theory  of  the  method  used  to  determine  the  re- 
actance of  the  armature  of  the  alternator. 

3.  Why  does  the  voltage  drop  more  rapidly  at  low  power  factors 
than  at  high  power  factors? 

4.  Why  are  alternators  rated  in  kilovolt-amperes  and  not  in 
kilowatts? 

5.  How  is  the  voltage  of  an  alternator  maintained  constant  in 
practice,  at  all  loads  and  power  factors? 

EXPERIMENT  17 

Object  of  Experiment. — To  start  up  a  synchronous  motor  and 
determine  its  running  characteristics. 

Reference.     Chap.  XXXIII,  page  252,  also  page  296. 

Connections. — Fig.  398,  page  258,  for  single  phase  machines. 
For  three  phase  machines  use  the  connection  in  Fig.  448. 

Method  of  Starting. — Start  up  the  synchronous  motor  by 
means  of  the  belted  direct- current  motor  M  and  adjust  the  speed 


ART.  404] 


LABORATORY  COURSE 


379 


of  the  synchronous  motor  and  its  excitation  until  the  voltage  E2 
is  equal  to  E\  and  the  synchronizing  lamps  all  remain  dark  for  a 
few  seconds  at  a  time.  (If  all  the  lamps  do  not  become  dark  at 
the  same  instant  then  interchange  two  of  the  motor  leads.) 

When  all  three  lamps  are  dark  at  the  same  instant,  then  close 
the  three  switches  Si  S2  and  $3  at  the  same  time.  The  starting 
motor  can  then  be  disconnected  by  throwing  the  belt,  and  the 
synchronous  motor  loaded  by  means  of  a  prony  brake. 

Readings. — With  the  applied  voltage  E2,  the  frequency,  and 
the  brake  readings  all  constant;  take  readings  of  current  I  and  of 
watts  (Wi+  Wz)  for  different  values  of  the  exciting  current  //  and 
for  three  different  settings  of  the  brake. 


FIG.  448. 


Curves. — Plot      armature      current,     and      power      factor 


/        watts 


- 


on  an  exciting  current  base. 


\volt  amperes/  ' 
Questions. — 1.  Explain  the  shapes  of  the  curves. 

2.  What  are  the  advantages  and  disadvantages  of  the  syn- 
chronous motor  compared  with  the  induction  motor? 

3.  What  effect  has  the  power  factor  of  the  load  on  the  size  of 
the  alternator  and  on  the  power  loss  in  the  transmission  line  supply- 
ing the  load? 

EXPERIMENT  18 

Object  of  Experiment. — To  determine  the  characteristics  of  a 
rotary  converter. 

Reference.     Page  318. 

Connection. — On  the  A.C.  end  the'machine  is  connected  in  the 
same  way  as  a  synchronous  motor,  see  experiment  17,  while  on  the 
D.C.  end  it  is  connected  in  the  same  way  as  a  shunt  generator. 


380     PRINCIPLES  OF  ELECTRICAL  ENGINEERING      [CHAP.  XLIII 

Method  of  Starting.  —  The  machine  may  be  started  up  as  a 
motor  from  the  D.C.  end  instead  of  by  means  of  a  starting  motor, 
but  it  must  be  synchronized  in  the  same  way  as  a  synchronous 
motor,  see  experiment  17,  before  it  is  connected  to  the  A.C. 
mains. 

Readings.  —  With  the  applied  voltage  E2,  the  frequency,  and 
EZ  Is  the  output  of  the  direct  current  side  all  constant,  take 
readings  of  the  current  7  and  of  the  watts  W  for  different  values 
of  the  exciting  current  //.  Take  a  complete  set  of  readings  for 
1/4,  1/2,  3/4  and  full  load  output  from  the  direct  current  side. 

Curves.—  Plot       armature       current,      and      power     factor 


\ 


,  on  an  exciting  current  base. 

' 


volts  amperes/ 

Plot  applied  voltage  E2  and  direct  voltage  E3  on  a  direct 
current  (/„)  base. 

Plot  efficiency  on  a  kilowatt  output  base  at  100  per  cent,  and 
at  some  other  power  factor. 

Questions.  —  1.  Answer  questions  1  and  3  in  experiment  17  if 
that  experiment  was  not  performed. 

2.  Why  is  the  voltage  of  the  direct  current  side  independent 
of  the  field  current,  and  how  can  this  voltage  be  controlled? 

EXPERIMENT  19 

Object  of  Experiment.  —  To  determine  the  starting  and  the 
running  characteristics  of  a  polyphase  induction  motor. 

References.    Chaps.  XXXVI  and  XXXVII,  page  283. 

Connections.  —  The  student  should  draw  out  a  diagram  of 
connections  and  specify  the  range  of  the  instruments  required. 

Readings.  —  To  show  the  effect  of  increase  in  applied  voltage 
on  the  current,  torque  and  power  factor  at  starting  (start  with 
a  low  value  of  applied  voltage). 

Also  readings  to  show  the  variation  of  efficiency,  power  factor, 
current,  and  speed  with  horse-power  output,  the  applied  voltage 
and  frequency  being  normal  and  constant. 

Curves.  —  Draw  the  curves  necessary  to  show  the  characteristics 
clearly. 

Questions.  —  1.  Why  is  the  current  for  a  given  torque  greater 
when  the  motor  is  at  standstill  than  when  running? 

2.  What  are  the  advantages  and  disadvantages  of  the  squirrel 
cage  induction  motor  compared  with  the  wound  rotor  induction 
motor? 


ART.  404]  LABORATORY  COURSE  381 

3.  What  are  the  relative  advantages  and  disadvantages  of 
the  squirrel  cage  induction  motor  and  the  synchronous  motor? 

4.  Explain  the  shape  of  the  power  factor  curve.     Why  is  the 
power  factor  low  at  light  loads? 

5.  Explain  the  shape  of  the  speed  curve. 

6.  How  would  you  reverse  the  direction  of  rotation  of  an 
induction  motor? 

7.  How  can  the  speed  of  an  induction  motor  be  varied  for  a 
given  load?     How  does  the  induction  motor  compare  with  the 
direct  current  shunt  motor  for  the  operation  of  machine  tools, 
and  with  the  direct  current  series  motor  for  the  operation  of 
cranes? 

8.  What  effect  has  an  increase  or  decrease  of  applied  voltage 
on  the  speed  of  an  induction  motor  at  no-load  and  at  full-load? 
Try  this  experiment. 

EXPERIMENT  20 

Object  of  Experiment. — To  determine  the  voltage  and  current 
relations  with  different  transformer  connections. 

References.— Pages  233  to  238,  also  Chap.  XXXV. 
Connections.— a  F  to  F,  Fig.  322,  page  276. 
b  Y  to  delta 
c  delta  to  delta 

d  Scott  connection  to  obtain  two  phase  from 
3  phase. 


FIG.  449. 


Note. — Each  transformer  should  be  protected  with  fuses  in 
case  of  short  circuit.  If  for  example  the  third  transformer  in  a 
delta  connected  bank  was  connected  as  in  diagram  B,  Fig.  449, 
instead  of  as  in  diagram  A,  there  would  be  a  large  voltage  be- 
tween points  Si  and  $3,  and  if  they  were  joined  together  a  very 
large  current  would  flow  through  the  closed  circuit. 

Readings. — Measure  the  voltage  per  phase,  the  voltage  be- 
tween lines,  the  current  in  each  transformer  and  the  current  in 


382      PRINCIPLES  OF  ELECTRICAL  ENGINEERING      [CHAP.  XLII 

the  lines,  and  compare  the  values  obtained  with  the  theoretical 
values. 

Question. — 1.  What  are  the  advantages  and  disadvantages 
of  the  Y  and  delta  connections? 

2.  Show  by  experiment  that  three  phase  power  can  be  ob- 
tained from  a  delta  connected  bank  if  one  transformer  is  removed, 
but  cannot  be  obtained  from  a  F-connected  bank  after  the 
removal  of  one  transformer. 


INDEX 


Adjustable     speed     operation,     in-      Arc  welding  generator,  190 


duction  motors,  293,  298, 
300 

series  motors,  92 

shunt  motors,  89,  105-112 
Air  blast  transformers,  271 
Air  compressors,  drive  for,  296 
Air  gap,  direct  current  machines,  48, 
68 

induction  motors,  292 
All  day  efficiency,  269 
Alternating  current,  48,  197 
Alternator,    armature    reaction    of, 
244 

characteristics,  244 

construction,  229,  239 

efficiency,  250 

excitation,  240 

inductor  type,  241 

magneto,  242 

parallel  operation  of,  259 

rating,  251 

reactance,  246 

regulation,  245,  248 

revolving  armature  type  240 

revolving  field  type  229,  239 

simple,  191 

single-phase,  231 

three-phase,  231 

two-phase,  230 

vector  diagram,  245 
Aluminium  arrester,  344 
Amalgamation  of  zinc,  143 
Ammeter,  8,  19,  198 
Ammeter  shunt,  368 
Ampere,  5,  18 
Ampere-hour,  18,  33 
Ampere-hour  efficiency,  153,  161 
Ampere-turn,  33 
Anode,  139 
Arc  lamps,  354,  365 
Arc  light  generator,  75 


Armature,  49  58, 
Armature  copper  loss,  95 

core,  55 

reaction,  67,  83,  89,  244,  312 

resistance  drop,  72,  79 

windings,  48,  230 
Arrester,  lightning,  343 
Automatic  current  regulator,  76 

feeder  regulator,  281 

motor  starter,  130,  133,  306 

switch,  183 

voltage  regulator,  74,  249 
Automobile  batteries,  149,  151,  153 

lighting,  186 

Autotransformer,  279,  301 
Average  current,  197 
Axle  generator,  182 

Back  e.m.f.,  79,  253,  261,  311 
Balanced  load,  238,  316 
Balancer,  316 
Battery  capacity,  153,  155,  175 

characteristics,  152,  160 

construction,  148,  158 

control,  171-179 

efficiency,  153,  161 

electromotive  force,  141,  142, 
151,  161 

primary,  140-145 

resistance,  141,  152,  161 

storage,  146-161 

temperature,  155,  161 
Blow-out  coil,  115,  123,  129 
Booster,  173,  176,  180,  315 
Boosting  transformer,  281 
Boring  mill,  motor  for,  108 
Brake,  46,  333 
Brake  test,  98 
Braking,  dynamic,  333 
Brushes,  50,  58 
Brushes,  carbon,  66 


383 


384 


INDEX 


Brushes,  resistance  of,  63,  66 

shifting  of,  63,  83 
Bucking  field  coils,  186 
Building  up  of  voltage,  71 

C.A.V.  generator,  189 

Cables,  underground,  347 

Candle-power,  354,  365 

Capacity  circuits,  218 

Capacity  of  a  battery,  153,  155,  175 

Capacity  reactance,  221 

Car  lighting,  181 

Carbon  battery  regulator,  176 

brushes,  66 

incandescent  lamp,  352 

lamp  regulator,  170,  182 

pile  rheostat,  29 

switch  contacts,  114 
Cast  iron  grid  resistance,  27 
Cement  mill  motors,  297 
Characteristics,  alternator,  244 

battery,  152,  160 

d.c.  generator,  70-77 

d.c.  motor,  85-94 

induction  motor,  290,  292 

single-phase  motor,  310,  311 
Charge  of  a  condenser,  218 
Choke  coil,  343 
Circuit,  breaker,  39,  117,  170 

formulae,  223 

magnetic,  33,  46 
Circuits,  electric  and  hydraulic,  18 

parallel  and  series,  22,  212,  216, 

224,  226 
Circular  mil,  20 
Circulating  oil  method  of  cooling, 

272 

Clutch,  electromagnetic,  46 
Coefficient  of  adhesion,  323 

of  self  induction,  206 
Color  of  light,  362 
Commutation,  62,  68,  83 

limit  of  output,  100 

of  a.c.  motors,  314 
Commutator,  50,  58 
Compensating  winding,  312 
Compensator,  starting,  302 
Compound  excitation,  61 

generator,  74,  166 


Compound  motor,  93 

starter,  120 
Condensers,  218,  222 
Constant  current  generator,  75 

regulator,  76 

transformer,  267 
Contactor  switch,  116,  132 
Contacts,  carbon,  114 
Control,  of  batteries,  171-179 

of  railway  motors,  126,  133,  327 

multiple  unit,  133 

series  parallel,  126 
Controllers,  122,  124 

automatic,  130 

crane,  122 

magnetic  switch,  131 

master,  132 
Conversion  factors,  15 
Converter,  ^mercury  vapor, ,  357 

rotary,  318 
Cooling  of  machines,  99,  100 

transformers,  270 
Copper  losses,  95 
Core  depth,  49,  56 

laminations,  55 
Core  type  transformers,  279 
Corkscrew  law,  5 
Cost  of  motors,  101,  108 
Coulomb,  18 
Coulomb's  law,  1 
Counter  e.m.f.,  f9 
Crane  motors,  92,  103,  298,  332 
Cross  magnetizing  effect,  67,  83,  312 
Current,  alternating,  48,  197,  212 

direct,  48  ' 

direction  of,  4 

effective,  197,  212 

unit,  5 
Current  carrying  capacity  of  wires, 

368 

Current  transformer,  351 
Cycle,  194 

Dampers,  295 

Daniell  cell,  141 

Delta  connection,  233,  236,  276,  278, 

Demagnetizing  effect,  68,  83 

Dielectric  strength,  22 

Differential  booster,  176 


INDEX 


385 


Direct  current,  48 

Direct  current  generator,  see  Gene- 
rators 
Direction  of  current,  4 

e.m.f.,  9,  11 

force  on  a  conductor,  7 

magnetic  field,  2 
Disconnecting  switch,  345 
Driving  force  of  a  motor,  78 
Drop  in  armature,  72,  79 

in  transmission  lines,  23 
Drum  type  controllers,  124 

windings,  52 
Dry  cells,  143 
Dynamic  braking,  333 

Eddy  current  loss,  96,  269 
Edison  battery,  157 
Edison  Lalande  cell,  144 
Effective  current,  197,  212 
Efficiency,  97 

alternator,  250 

battery,  152,  161 

direct-current  machines,  98 

illuminants,  360 

induction  motor,  299 

rotary  converter,  320 

transformer,  268 

values  of,  98 
Electric  furnace,  74 

hammer,  38 

locomotive,  332 

welder,  264 
Electrical  degrees,  200 

energy,  14 

power,  14 
Electrodynamometer  instruments, 

199 

Electrolysis,  139 
Electrolyte,  139,  156,  160 
Electromagnet,  5,  37 
Electromagnetic  brakes,  46,  333 

clutches,  46 

induction,  9 

motor,  41 
Electromotive  force,  back,  79 

direction  of,  9,  11 

generation  of,  9,  11 

of  a  battery,  141,  142,  151,  161 


Electromotive  force,  of  self  induc- 
tion, 12, 
unit  of,  9 

Elevator  motors,  103 

Enclosed  arc  lamps,  355 

Enclosed  machines,  100 

End  cell  control  of  batteries,  172 

Energy,  chemical  and  electrical,  140 
heat  and  electrical,  15 
mechanical  and  electrical,  14 
required  for  an  electric  car,  326 

Equalizer  connection,  166 

Ewing's  theory  of  magnetism,  36 

Excitation,  59,  71 

Exciter  for  alternators,  240 

Exciting  current,  60 

External  characteristics,  71 

Fan  drive,  111,  112,  296,  300 

Farads,  219 

Farm  house  lighting,  169 

Feeder  regulator,  281,  339 

Field  copper  loss,  95 

Field,  magnetic,  1-5,  32 

Field,  reversing,  65,  68,  83 

Field  rheostat,  74 

Flame  arc  lamp,  356 

Flashing  at  switches,  12 

Flat  compounding,  75 

Fleming's  rule,  9 

Float  switch  control,  130 

Floating  battery,  154,  179 

Flux  density,  3,  33,  44 

Flywheel  motor  generator  set,  335 

Flywheel,  use  of ,  94,  104 

Force  on  a  conductor,  6 

Formula)  for  circuits,  223 

Frequency,  194,  341 

natural,  226 

of  resonance,  226 
Frequency  changer,  321 

meter,  195 

Furnace,  induction,  263,  267 
Fuses,  117 

Gas-filled  lamp,  353 
Gassing  of  batteries,  147 
Generator,  axle  driven,  182 

car  lighting,  181 

constant  current,  75 


386 


INDEX 


Generator,  direct-current,  armature 

reaction,  67 
characteristics,  71 
commutation,  62,  68 
compound,  74 
construction,  56 
excitation,  60 
limits  of  output,  100 
parallel  operation,  164,  166 
regulation,  71 
series,  75 
shunt,  72 
windings,  48 
induction,  294 
retarding  force  of,  78 
three-wire,  317 
variable  speed,  181 
Glare,  363 

Gramme  ring  winding,  48 
Grid  resistance,  27 
Growth  of  current,  12 

Hammer,  electric,  38 

Head  and  end  system,  181 

Heat  and  electrical  energy,  15 

Heater  units,  27 

Heating  of  machines,  99,  100,  239 

of  transformers,  270 
Henry,  207 
Hoist  motor,  332 
Hoisting,  332 
Holding  magnet,  43 
Horn  gap  arrester,  345 

switch,  116 
Horse-power,  14 
Horse-power-hour,  15 
Hunting,  258,  295 
Hydrometer,  156 
Hysteresis,  36 

loss,  95,  269 

Ignition,  make  and  break,  206 
llluminants,  efficiency,  360 
Illumination,  principles,  362 

intensity,  364 
Incandescent  lamps,  352 
Inductance,  205,  209 

adjustable,  210 

of  transmission  line,  211,  216 


Induction,  electromagnetic,  9 
furnace,  263,  267 
generator,  294 

motor,  adjustable  speed  opera- 
tion, 293,  298 
applications,  296 
characteristics,  291 
construction,  283 
efficiency,  290,  299 
for  railway  service,  327 
power  factor,  293 
single-phase,  308 
slip,  290 

speed,  287,  290,  293 
squirrel  cage,  283,  296 
starters,  301-307 
starting  torque,  287,  297 
vector  diagram,  291 
wound  rotor,  288,  297 
mutual,  11 
regulator,  281 
self,  11,  206 
Inductive  circuit,  207,  210 

reactance,  208 
Inductor  alternator,  241 
Instrument  transformers,  351 
Insulating  materials,  22,  26,  32,  99 
Insulation  of  windings,  58 
Insulators,  line,  346 
Intensity  of  illumination,  364 

of  magnetic  field,  2,  3 
Intermittent  ratings,  101 
Internal  resistance  of  a  battery,  141, 

152,  161 

Interpole  machines,  65,  100 
Ions,  139 
Iron  loss,  96 

Iron,  magnetic  properties,  32 
Ironclad  solenoids,  41 
Isolated  lighting  plants,  169 

Joules,  14 

Kathode,  139 
Kilovolt-ampere,  251 
Kilowatt,  14 
Kilowatt-hour,  15 
Knife  switches,  114 


INDEX 


387 


Lagging  current,  203,  208 
Lamination  of  armature  core,  55 

transformer  core,  269 
Lamps,  arc,  354 

connection  of,  365 

incandescent,  352 

mercury  vapor,  359 
Lamp  circuit  regulator,  170,  182 
Lathes,  drive  for,  108,  299 
Lead  battery,  146 
Leading  current,  203,  221 
Leakage  reactance  of  transformers, 

264 

Leclanche  cell,  143 
Left  hand  rule,  7 
Lenz's  law,  10 
Lifting  magnets,  43 
Light,  quality  of,  363 

unit  of,  354 
Lighting  of  cars  and  vehicles,  181 

country  roads,  363,  364 

drawing  offices,  363 

farm  houses,  169 

reading  rooms,  362 

streets,  360,  364 
Lightning  arresters,  343 

protection,  346 

Limits  of  output,  100,  255,  291 
Line  construction,  346 
Line  shaft  drive,  102,  297 
Lines  of  force,  2,  3 
Liquid  rheostat,  29,  335 
Loading  back  tests,  163 
Local  action,  143 
Locomotive,  electric,  332 
Long  shunt,  61 
Losses  in  alternators,  250 

direct-current  machines,  95 

transformers,  268 

transmission  lines,  215 
Lumens,  364 
Luminous  arc  lamps,  357 

Machine  toofdrive,  108,  299 
Magnet,  1 

alternating  current,  306 

electro,  5,  37 

lifting,  43 

permanent,  35,  59 


Magnet,  pull  of,  40 
Magnetic,  brakes,  46,  333 
circuit,  33,  46 
clutches,  46 
field,  1-5,  32 
flux  density,  3,  33,  44 
hammer,  38 
properties  of  iron,  32 
separator,  47 
switch  controller,  131 
Magnetism,  molecular  theory  of,  36 

residual,  35,  70 
Magnetization  curves,  34,  70 
Magnetizing    current,    induction 

motor,  292 
transformer,  262 
Magneto,  59,  242 
Magnetomotive  force,  34 
Make  and  break  ignition,  206 
Manganin,  21 
Manholes,  347 
Master  controller,  132 
Maximum   output  of  direct-current 

machines,  100 
induction  motors,  291 
synchronous  motor's,  255 
Maxwells'  formula,  44 
Mercury  vapor  converter,  357 

lamp,  359 

Mechanical  losses,  95 
Mil,  circular,  20 
Molecular  magnets,  36 
Motor,  direct-current,  armature  re- 
action, 83,  89 
commutation,  83 
compound,  93 
driving  force,  78 
electromagnetic,  41 
limits  of  output,  100 
railway,  327 
series,  90 
shunt,  85 
speed,  80,  82 
starters  for,  87,  117,  130 
theory  of  operation,  80 
variable  speed,  89 
induction,  see  induction  motor 
repulsion,  313 
single-phase  induction,  308-310 


388 


INDEX 


Motor,  single  phase  series,  310,  313, 
327 

synchronous,    see    synchronous 

motor 

Motor    applications    to    air    com- 
pressors, 296 

boring  mills,  108 

cement  mills,  297 

cranes,  92,  103,  298,  332 

crushers,  94 

elevators,  103 

fans,  111,  112,  296,  300 

lathes,  108,  299 

line  shafts,  102,  297' 

machine  tools,  108,  300 

printing  presses,  111,  113 

pumps,  103,  296 

shears  and  punch  presses,  103, 
299 

textile  machinery,  298 

wood-working    machines,     102, 

297 

Motor  car  lighting,  186 
Motor  car  trains,  332 
Motor  generator  sets,  315,  335 
Multiple  switch  starter,   119,    13T, 
305 

unit  control,  133 

voltage  system,  109 
Multipolar  machines,  51,  56,  287 
Mutual  induction,  11 

Natural  frequency,  226 

Neutral  line,  62 

No-load  saturation  curve,  70 

No-voltage  release,  87, 125,  302,  304 

Non-arcing  metal,  343 

Non-inductive  resistance,  211 

Ohm's  law,  20 

Oil  circulation  for  transformers,  272 

Oil  for  transformers,  270 

Oil  switch,  300,  345 

Open  delta  connection,  277 

•Open  machines,  100 

Oscillograph,  193 

Overcompound  generator,  75 

Overhead  line  construction,  346 


Overload  relay   on  oil  switch,  345, 

350 
release,  118,  125,  304 

Parallel  circuits,  22,  216,  226 

Parallel  operation  of  alternators,  259 
direct-current    generators,    164, 

166 
rotary  converter,  320 

Pasted  plates  for  a  battery,  149 

Permanent  magnets,  35,  59 

Permeability,  33 

Phase,  relation,  203 

single-,  two-,  and  three-,  231 

Pin  insulators,  346 

Plante  plates,  148 

Polarization,  141 

Polyphase  circuits,  238 
rotary  converter,  320 

Potential  transformer,  351 

Power,  14 

in  a  capacity  circuit,  223 
in  an  inductive  circuit,  209 
in  a  resistance  circuit,  21,  211 
in  a  three-phase  circuit,  236,  237 
measurement  of,  214,  238    . 

Power  factor,  213 

correction,  256,  320 
of  an  induction  furnace,  267 
of  an  induction  motor,  293 
of  a  synchronous  motor,  256 
of  a  transformer,  262 

Power  station,  171,  338 

Primary  of  a  transformer,  261 

Printing  press  drive,  111,  113 

Pull  of  magnets,  40,  43 
solenoids,  37 

Pump  drive,  103,  296 

Punch  press  drive,  103,  299 

Puncture  of  insulation,  22 

Quantity  of  electricity,  18,  218 
Quick  break  switch,  114,  300 

Railway  motors,  327 
Rating  of  machines,  101,  251 
Ratio  of  transformation,  262 
of  rotary  converter,  318 


INDEX 


389 


Reactance,  capacity,  221 
inductive,  208 
of  an  alternator,  246 
of  a  transformer,  264 
of     a    transmission     line,  211, 

216 

Reflectors,  359 
Regulation  curves,  71 

of  alternators,  245,  248 

of     direct-current      generators, 

71-75 

of  a  transmission  line,  215 
speed,  107,  113,  293 
Regulator,  axle  generator,  184,  187 
constant  current,  76 
feeder,  281,  339 
lamp  circuit,  170,  182 
voltage,  74,  249 
Reluctance,  33 
Repulsion  motor,  313 
Residual  magnetism,  35,  70 
Resistance,  19 

battery  internal,  141,  152,  161 
brush  contact,  63,  66 
control  of  batteries,  171 
drop  in  armature,  72,  79 
for    adjustable     speed     motor, 

112,  300 

power  loss  in,  21,  211 
specific,  20 
starting  for   motor,  85,  87,  89, 

118,  305 
steadying,  365 

temperature  coefficient  of,  21 
Resistors,  26 
Resonance,  224,  227 
Reverse  current  circuit  breaker,  170 
Reversing  drum,  129 
Reversing  field,  65,  68,  83 
Revolving  field  alternator,  192,  229 

•    239 

of  an  induction  motor,  284 
Rheostats,  25 

carbon  pile,  29 
cast-iron  grid,  27 
field  circuit,  74 
liquid,  29,  335 
Right  hand  rule,  9,  192 
Ring  winding,  48 


Rosenberg  machine  ,188 
Rotary  converter,  318 
Rotor,  229 

induction  motor,  283,  288 

Safety  devices,  337 
Saturation  curve,  70 
magnetic,  46,  71 
Scott  connection,  274 
Searchlight  generator,  190 
Secondary  of  a  transformer,  261 
Self-cooled  transformer,  270 
Self  excitation,  60,  71 
Self  induction,  11,  206 
Self-starting     synchronous     motor 

294,  296 

Semi-enclosed  machines,  100 
Separately  excited  machines,  60,  71 
Separator,  magnetic,  47 
Series  arc  lighting,  366 
booster,  180 
circuits,  22,  212,  224 
excitation,  60 
generators,  75 
motors,  90,  310 
parallel  control,  126 
shunt,  77,  167 
Shades  for  lamps,  359 
Shadows,  363 

Shears,  motor  for,  103,  299 
Shell  type  transformers,  279 
Shifting  of  brushes,  63,  83 
Short-circuit  curve,  246 
Short  shunt,  61 
Shunt  excitation,  60 

generators,  72,  162,  164 
motors,  85,  105 
Single-phase,  231 
motors,  308-314 
railway  system,  329 
Sliding  contact  starter,  117 
Slip  of  induction  motors,  290,  293 
Slipping  belt  generator,  185 

clutch  generator,  186 
Slow  speed  drive,  113 
Solenoid,  32,  37,  41 
brake,  333 
starter,  130,  306 
Sparking,  at  a  commutator,  63 


390 


INDEX 


Sparking,  at  a  switch,  12 

limit  of  output,  100 
Specific  gravity  of  electrolyte,  156, 
157 

inductive  capacity,  219 

resistance,  20 
Speed,  adjustable,  89,  92,  105-112, 

293,  298,  300 
Speed  of  a  motor,  80-83,  89,  101 

of  an  induction  motor,  287,  290, 

293,  298 

of  a  series  motor,  92,  312 

of  a  shunt  motor,  89,  105-112 

of  a  synchronous  motor,   252, 
296 

regulation,  107,  113,  293 

regulators,  121 

synchronous,  252 

variation  by  armature  control, 

105,  112 

by  field  control,  107 
Speed  time  curve,  323 
Spider,  59 

Split-phase  method  of  starting,  308 
Split-pole  rotary  converter,  320 
Squirrel-cage  motor,  283,  296 
Star-delta  starter,  304 
Starters,  automatic,  130,  133,  306 

hand-operated,     87,     117-121, 

301-305,  340 

Starting  compensator,  302 
Starting    torque    of    an    induction 
motor,  287,  297 

of  a  series  motor,  90 

of  a  shunt  motor,  85 

of  a  synchronous  motor,   252, 

294,  296 

Starting  resistance,  85,  87,  89,  118, 

305 

Stator,  229,  283 
Steadying  resistance,  365 
Stiff  field,  69 
Stone  generator,  185 
Storage  battery,  see  battery,   146- 

161 

Stray  loss,  96 
Street  car  controller,  126 
Street  lighting,  360,  364 
Substation,  339 


Sulphation,  147 

Sum  of  alternating  voltages,  203 
Suspension  insulator,  346 
Switches,  automatic  battery,  183 

carbon  break;  114 

contactor,  116,  132 

disconnecting,  345 

float,  130 

magnetic,  131 

oil,  300,  345 

quick-break,  114 

sparking  at,  12 
Switchboards,  348 
Symbols  for  alternating  currents,  198 
Synchronizing,  258,  378 
Synchronous  motor,  252-259,  294- 
296 

applications,  296 

dampers  for,  295 

hunting,  258,  295 

power  factor,  256 

self-starting,  294,  296 

speed,  252,  296 

starting  torque,  252,  294,  296 

synchronizing,  258,  378 

vector  diagram,  254 
Synchronous  speed,  252 
Synchroscope,  258,  350 

Temperature  coefficient  of  resistance, 

21 

of  a  battery,  155,  161 
of  motors,  99,  100,  102 

Terminal  station,  339 

Textile  mill,  drive,  298 

Three-phase  alternator,  231 
connection  of  load,  237 
connection  of  transformers,  276 
transformers,  279 

Three-wire  generators,  317 
system,  316,  338 

Torque  of  a  motor,  82 

of  an  induction  motor,  287,  297 
of  a  series  motor,  90,  310 
of  a  single-phase  motor.  308,  310 
of  a  shunt  motor,  85,  88 
of  a  synchronous  motor,   252, 
294,  296 

Traction,  322-332 


INDEX 


391 


Tractive  effort,  322 
Train  lighting,  181 

friction,  322 
Transmission  line,  drop,  23 

inductance,  211,  216 

insulation,  346 

losses,  215 

regulation,  215 

single-  and  three-phase,  341 
Transmission,  underground,  347 

voltages,  338,  341 
Transformation  ratio,  262 
Transformer,  boosting,  281 

connections,  273-279,  381 

constant  current,  267 

cooling,  270 

efficiency,  268 

instrument,  351 

leakage  reactance,  264 

lighting,  273 

losses,  268 

theory  of  operation,  261 

vector  diagram,  263,  265 
Trolley  wire,  329 
Tungsten  lamp,  352 
Two-phase  alternator,  230 

connection      of      transformers, 
273 

Underground  cables,  347 
Unit  of  current,  5 

e.m.f.,  9 

light,  354 

pole  strength,  1 

power,  14 

work,  14 

V-connection,  277 
Variable  speed  generators,  182 
Variable  speed  operation,  induction 
motors,  293,  298,  300 

series  motors,  92 

shunt  motors,  89,  105-112 
Vector  diagram  for  alternators,  245 

induction  motors,  291 


Vector  diagram  for  parallel  circuit, 
217,  227 

series  circuit,  213,  225 

series  motor,  311 

synchronous  motor,  254 

transformer,  263,  265 

transmission  line,  215,  342 
Vector  sum,  203 
Vector  representation,  201 
Vehicle  lighting,  l'81 
Vibrating  contact  regulator,  187 
Volt,  9 

Volt  efficiency,  152,  161 
Voltage,  average,  197 

drop  in  transmission  lines,  23 

effective,  212 

for  arc  lamps,  367 

for  traction,  329 

for  transmission,  338,  341 

of  a  battery,  151,  160 

regulators,  74,  249 
Voltages  used  in  practice,  341 
Voltameter,  139 
Voltmeter,  19,  198 

Wagner  motor,  314 1 

Ward  Leonard  system,  110,  335 

Water  cooled  transformers,  271 

Water  rheostat,  29,  335 

Watts,  14 

Wattmeter,  214 

Wave  form,  193 

Welder,  electric,  264 

Windings,  48,  229 

Wire,    current    carrying     capacity, 

338,  348,  368 
Wood-working    machinery,    motors 

for,  102,  297 
Work,  unit  of,  14 
Wound  rotor  motor,  288 

Y-connection,    233,    235,    237,  276, 

278 

Y-delta  starter,  304 
Yoke  of  machine,  56,  58 


PINE  OF  25  CENTS 


1943 


-ree-14-w 

IvoTT^i 

fifia-y—  ___ 

LurSlfif 

_i  3^: 

—  —  _L___:  
^c 

27Aug5ll[! 


*-» 


FEB  12  1942 


i^tN' 


"RCcf5FWF 

•r 


LD  21-100w-7,'40 (6936s) 


* 


YC   19562 


UNIVERSITY  OF  CALIFORNIA  LIBRARY 


V  ; 
#4. 


